Add settings docs

docs/plugins/plugin-structure.md

@@ -20,6 +20,7 @@ This plugin will write to console all changes in your search field of Cerebro ap
   * `copyToClipboard` – `Function(text: String)`, copy text to clipboard;
   * `replaceTerm` – `Function(text: String)`, replace text in main Cerebro input;
   * `hideWindow` – `Function()`, hide main Cerebro window.
+* `settings` - `Object`, contains user provided values of all specified settings keys;
 
 
 Let's show something in results list:
@@ -149,3 +150,21 @@ Arguments: `message: Object` – object that you sent from `initializeAsync`
 Use this function to receive data back from your `initializeAsync` function. 
 
 Check `initializeAsync` and `onMessage` [example](./examples.md#using-initializeasync-and-onmessage)
+
+### `settings`
+Type: `Object`
+
+This object is used to specify settings that a plugin user can change. Each setting should include a `description` and a `type`. Other keys include:
+  * `label` - `String`, object key for the setting. also used to access it;
+  * `description` -  `String`, description of the setting;
+  * `type` - `String`, used to decide element for rendering a setting:
+    * `string`
+    * `number`
+    * `bool`
+    * `option`
+  * `defaultValue` - `Any`, default value for the setting;
+  * `options` - `Array`, all possible options that can be selected by the user. applicable only for `option`;
+  * `multi` - `Bool`, allows user to select more than one option for `option` settings;
+  * `createable` - `Bool`, allows user created options;
+
+Look at [React Select](https://github.com/JedWatson/react-select) for more details on how the `option` type works.