Make it clear that PR has to be opened in Element Android for now.

README.md

@@ -6,6 +6,10 @@ Matrix SDK for Android, extracted from the Element Android application.
 
 The SDK is still in beta, and replaces the [legacy Matrix Android SDK](https://github.com/matrix-org/matrix-android-sdk) provided by Matrix.org
 
+## Important notice
+
+<b>For now, this project is an extract of the Matrix SDK module from Element Android. Please do not open a pull request on this project. If you want to propose a change on the SDK, please open a PR on https://github.com/vector-im/element-android. Thanks!</b>
+
 ## About
 
 This repository contains the matrix-android-sdk extracted from the project [Element Android](https://github.com/vector-im/element-android)