Add documentation to explain how to install the application from a Github release.

docs/install_from_github_release.md

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+# Installing Element X Android from a Github Release.
+
+This document explain how to install Element X Android from a Github Release.
+
+<!--- TOC -->
+
+* [Requirements](#requirements)
+* [Steps](#steps)
+* [I already have the application on my phone](#i-already-have-the-application-on-my-phone)
+
+<!--- END -->
+
+## Requirements
+
+The Github release will contain an Android App Bundle (with `aab` extension) file, and not APKs like on the Element Android project. So there is some steps to perform to generate and signed the APKs from the App Bundle. The generated APKs will then be selected depending on your phone configuration to be installed on a device (emulator or real device).
+
+The easiest way to do that is to use the debug signature that is shared between the developers and stored in the project. So we recommend to checkout the project first. Android Studio is not required, you can use the command line.
+
+You will also need to install [bundletool](https://developer.android.com/studio/command-line/bundletool). On MacOS, you can run the following command:
+
+```bash
+brew install bundletool
+```
+
+## Steps
+
+1. Open the GitHub release that you want to install from https://github.com/vector-im/element-x-android/releases;
+2. Download the asset `app-release-signed.aab`
+3. Navigate to the folder where you cloned the project and run the following command:
+```bash
+bundletool build-apks --bundle=<PATH_TO_YOUR_APP-RELEASE-SIGNED.AAB_FILE> --output=./tmp/elementx.apks \
+      --ks=./app/signature/debug.keystore --ks-pass=pass:android --ks-key-alias=androiddebugkey --key-pass=pass:android \
+      --overwrite
+```
+For instance:
+```bash
+bundletool build-apks --bundle=./tmp/Element/0.1.5/app-release-signed.aab --output=./tmp/elementx.apks \ 
+      --ks=./app/signature/debug.keystore --ks-pass=pass:android --ks-key-alias=androiddebugkey --key-pass=pass:android \
+      --overwrite
+```
+4. Run an Android emulator, or connect a real device to your computer.
+5. Install the APKs on the device:
+```bash
+bundletool install-apks --apks=./tmp/elementx.apks
+```
+
+That's it, the application should be installed on your device, you can start it from the launcher icon.
+
+##  I already have the application on my phone
+
+If the application was already installed on your phone, there are several cases:
+
+- it was installed from the PlayStore, you will have to uninstall it first, because the signature will not match.
+- it was installed from a previous GitHub release, this is like an application upgrade, so no need to uninstall the existing app.
+- it was installed from a more recent GitHub release, you will have to uninstall it first.