Write documentation about UnifiedPush.

Introduction is inspired from #2743.

docs/unifiedpush.md

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+# UnifiedPush
+
+<!--- TOC -->
+
+* [Introduction](#introduction)
+* [Configuration in Element-Android and their forks](#configuration-in-element-android-and-their-forks)
+  * [Enabling and disabling the feature](#enabling-and-disabling-the-feature)
+    * [Override the configuration at runtime](#override-the-configuration-at-runtime)
+    * [Enabling the feature](#enabling-the-feature)
+    * [Disabling the feature](#disabling-the-feature)
+  * [Useful links](#useful-links)
+
+<!--- END -->
+
+## Introduction
+
+The recently started UnifiedPush project is an Android protocol and library for apps to be able to receive distributor-agnostic push notifications.
+
+The *Gplay* variant of Element Android use the UnifiedPush library to still receive push notifications from FCM, but also alternatively from other non-Google distributor systems that the user can have installed on their device. Currently available are Gotify, a server and app that receives push notifications via a websocket, NoProvider2Push, a peer-to-peer system, and others. This would make it possible to have push notifications without depending on Google services or libraries.
+
+The UnifiedPush library comes in two variations: the FCM-added version of the library, which basically comes with the FCM distributor built into the library (so a user doesn't need to do anything other than install the app to get FCM notifications), and the main version of the library, which doesn't come with FCM embedded (so a user has to separately install the FCM, Gotify, or other distributor as an app on their phone to get push notifications).
+
+These two versions of the library are used in the Google Play version and F-Droid version of the app respectively, to be able to publish an easy-to-use no-setup-needed version of the app to Google Play, and a version that doesn't depend on any Google code to F-Droid.
+
+## Configuration in Element-Android and their forks
+
+### Enabling and disabling the feature
+
+Allowing the user to use an alternative distributor can be changed in [Config](../vector-config/src/main/java/im/vector/app/config/Config.kt). The flag is named `ALLOW_EXTERNAL_UNIFIED_PUSH_DISTRIBUTORS`. Default value is `true`.
+
+#### Override the configuration at runtime
+
+On debug version, it is possible to override this configuration at runtime, using the `Feature` screen. The Feature is named `Allow external UnifiedPush distributors`.
+
+#### Enabling the feature
+
+This is the default behavior of Element Android.
+
+If `ALLOW_EXTERNAL_UNIFIED_PUSH_DISTRIBUTORS` is set to true, it allows any available external UnifiedPush distributor to be chosen by the user.
+- For Gplay variant it means that FCM will be used by default, but user can choose another UnifiedPush distributor;
+- For F-Droid variant, it means that background polling will be used by default, but user can choose another UnifiedPush distributor.
+- On the UI, the setting to choose an alternative distributor will be visible to the user, and some tests in the notification troubleshoot screen will shown.
+- For F-Droid, if the user has chosen a distributor, the settings to configure the background polling will be hidden.
+
+#### Disabling the feature
+
+If `ALLOW_EXTERNAL_UNIFIED_PUSH_DISTRIBUTORS` is set to false, it prevents the usage of external UnifiedPush distributors.
+- For Gplay variant it means that only FCM will be used;
+- For F-Droid variant, it means that only background polling will be used.
+- On the UI, the setting to choose an alternative distributor will be hidden to the user, and some tests in the notification troubleshoot screen will be hidden.
+
+### Useful links
+
+- UnifiedPush official website: [https://unifiedpush.org/](https://unifiedpush.org/)
+- List of available distributors can be retrieved here: [https://unifiedpush.org/users/distributors/](https://unifiedpush.org/users/distributors/)
+- UnifiedPush project discussion can occurs here: [#unifiedpush:matrix.org](https://matrix.to/#/#unifiedpush:matrix.org)