Update README.md

README.md

@@ -29,6 +29,11 @@ our [Android web views SDK guide](doc/web_views.md).
       * [In-App Purchase verification](#iap-verification)
       * [Callback parameters](#callback-parameters)
       * [Partner parameters](#partner-parameters)
+   * [Session parameters](#session-parameters)
+      * [Session callback parameters](#session-callback-parameters)
+      * [Session partner parameters](#session-partner-parameters)
+      * [External device id](#external-device-id)
+      * [Delay start](#delay-start)
    * [Attribution callback](#attribution-callback)
    * [Session and event callbacks](#session-event-callbacks)
    * [Disable tracking](#disable-tracking)
@@ -367,7 +372,7 @@ future. Depending on your coding style you might want to implement this in a com
 
 ### <a id="adjust-logging"></a>Adjust Logging
 
-You can increase or decrease the amount of logs you see in tests by calling `setLogLevel` on your `AdjustConfig` instance 
+You can increase or decrease the amount of logs that you see during testing by calling `setLogLevel` on your `AdjustConfig` instance 
 with one of the following parameters:
 
 ```java
@@ -419,7 +424,7 @@ choice. Read more about [currency conversion here.][currency-conversion]
 
 You can read more about revenue and event tracking in the [event tracking guide.][event-tracking]
 
-The event instance can be used to configure the event even more before tracking it.
+The event instance can be used to configure the event further before tracking it:
 
 #### <a id="iap-verification">In-App Purchase verification
 
@@ -430,7 +435,7 @@ receipt verification tool, then check out our Android purchase SDK to read more
 #### <a id="callback-parameters">Callback parameters
 
 You can register a callback URL for your events in your [dashboard]. We will send a GET request to that URL whenever the 
-event gets tracked. You can add callback parameters to that event by calling `addCallbackParameter` on the event instance 
+event is tracked. You can add callback parameters to that event by calling `addCallbackParameter` to the event instance 
 before tracking it. We will then append these parameters to your callback URL.
 
 For example, suppose you have registered the URL `http://www.adjust.com/callback` then track an event like this:
@@ -452,15 +457,14 @@ http://www.adjust.com/callback?key=value&foo=bar
 
 It should be mentioned that we support a variety of placeholders like `{gps_adid}` that can be used as parameter values. 
 In the resulting callback this placeholder would be replaced with the Google Play Services ID  of the current device. Also 
-note that we don't store any of your custom parameters, but only append them to your callbacks. If you haven't registered a
-callback for an event, these parameters won't even be read.
+note that we don't store any of your custom parameters, but only append them to your callbacks. If you haven't registered a callback for an event, these parameters won't even be read.
 
 You can read more about using URL callbacks, including a full list of available values, in our 
 [callbacks guide][callbacks-guide].
 
 #### <a id="partner-parameters">Partner parameters
 
-You can also add parameters to be transmitted to network partners, for the integrations that have been activated in your 
+You can also add parameters to be transmitted to network partners, which have been activated in your 
 adjust dashboard.
 
 This works similarly to the callback parameters mentioned above, but can be added by calling the `addPartnerParameter` 
@@ -477,6 +481,102 @@ Adjust.trackEvent(event);
 
 You can read more about special partners and these integrations in our [guide to special partners.][special-partners]
 
+### <a id="session-parameters">Set up session parameters
+
+Some parameters are saved to be send in every event and session of the adjust SDK.
+Once you have add any of these parameters, you don't need to add them every time, since they will be saved locally.
+If you add the same parameter twice, there will be no effect.
+
+These session parameters can be called before the adjust SDK is launched to make sure they are send even on install.
+If you need to send them with an install, but can only obtain the needed values after launch, it's possible to [delay](#delay-start) the first launch of the adjust SDK to allow this behaviour.
+
+#### <a id="session-callback-parameters"> Session callback parameters
+
+The same callback parameters that are registered for [events](#callback-parameters) can be also saved to be sent in every event or session of the adjust SDK.
+
+The session callback parameters have a similar interface of the event callback parameters. 
+Instead of adding the key and it's value to an event, it's added through a call to `Adjust.addSessionCallbackParameter(String key, String value)`:
+
+```java
+Adjust.addSessionCallbackParameter("foo", "bar");
+```
+
+The session callback parameters will be merged with the callback parameters added to an event.
+The callback parameters added to an event have precedence over the session callback parameters. 
+Meaning that, when adding a callback parameter to an event with the same key to one added from the session, the value that prevails is the callback parameter added to the event.
+
+It's possible to remove a specific session callback parameter by passing the desiring key to the method `Adjust.removeSessionCallbackParameter(String key)`.
+
+```java
+Adjust.removeSessionCallbackParameter("foo");
+```
+
+If you wish to remove all key and values from the session callback parameters, you can reset it with the method `Adjust.resetSessionCallbackParameters()`.
+
+```java
+Adjust.resetSessionCallbackParameters();
+```
+
+#### <a id="session-partner-parameters"> Session partner parameters
+
+In the same way that there is [session callback parameters](#session-callback-parameters) that are send every in event or session of the adjust SDK, there is also session partner parameters.
+
+These will be transmitted to network partners, for the integrations that have been activated in your adjust [dashboard].
+
+The session partner parameters have a similar interface of the event partner parameters. 
+Instead of adding the key and it's value to an event, it's added through a call to `Adjust.addSessionPartnerParameter(String key, String value)`:
+
+```java
+Adjust.addSessionPartnerParameter("foo", "bar");
+```
+
+The session partner parameters will be merged with the partner parameters added to an event.
+The partner parameters added to an event have precedence over the session partner parameters. 
+Meaning that, when adding a partner parameter to an event with the same key to one added from the session, the value that prevails is the partner parameter added to the event.
+
+It's possible to remove a specific session partner parameter by passing the desiring key to the method `Adjust.removeSessionPartnerParameter(String key)`.
+
+```java
+Adjust.removeSessionPartnerParameter("foo");
+```
+
+If you wish to remove all key and values from the session partner parameters, you can reset it with the method `Adjust.resetSessionPartnerParameters()`.
+
+```java
+Adjust.resetSessionPartnerParameters();
+```
+
+#### <a id="external-device-id"> External device id
+
+The adjust SDK uses device ids that are available from iOS, but your app might have other sources of identifiers that can help features like ad conversion.
+
+To save your unique identifier in the session parameters that are send in every request and session, call the `Adjust.addExternalDeviceId(String externalDeviceId)` method:
+
+```java
+Adjust.addExternalDeviceId("customDeviceId");
+```
+
+It's also possible to remove the saved unique identifier by calling the method `Adjust.resetExternalDeviceId()`:
+
+```java
+Adjust.resetExternalDeviceId();
+```
+
+#### <a id="delay-start"> Delay start
+
+Delaying the start of the adjust SDK allows your app some time to obtain session parameters, such as unique identifiers, to be send on install.
+
+Set the initial delay time in seconds with the method `setDelayStart` in the `AdjustConfig` instance:
+
+```java
+adjustConfig.setDelayStart(5.5);
+```
+
+In this case this will make the adjust SDK not send the initial install session and any event created for 5.5 seconds.
+After this time is expired or if you call `Adjust.sendFirstPackages()` in the meanwhile, every session parameter will be added to the delayed install session and events and the adjust SDK will resume as usual.
+
+The maximum delay start time of the adjust SDK is 10 seconds.
+
 ### <a id="attribution-callback"></a>Attribution callback
 
 You can register a listener to be notified of tracker attribution changes. Due to the different sources considered for 
@@ -507,7 +607,7 @@ config.setOnAttributionChangedListener(this);
 Adjust.onCreate(config);
 ```
 
-The listener function will be called when the SDK receives the final attribution information. Within the listener function 
+The listener function will be called after the SDK receives the final attribution data. Within the listener function 
 you have access to the `attribution` parameter. Here is a quick summary of its properties:
 
 - `String trackerToken` the tracker token of the current install.
@@ -577,7 +677,7 @@ Both event response data objects contain:
 
 And both event and session failed objects also contain:
 
-- `boolean willRetry` indicates there will be an attempt to resend the package at a later time.
+- `boolean willRetry` indicates that will be an attempt to resend the package at a later time.
 
 ### <a id="disable-tracking"></a>Disable tracking
 
@@ -604,7 +704,7 @@ Adjust.setOfflineMode(true);
 ```
 
 Conversely, you can deactivate offline mode by calling `setOfflineMode` with `false`. When the adjust SDK is put back in 
-online mode, all saved information is send to our servers with the correct time information.
+online mode, all saved information is sent to our servers with the correct time information.
 
 Unlike disabling tracking, this setting is **not remembered** between sessions. This means that the SDK is in online mode 
 whenever it is started, even if the app was terminated in offline mode.