Add new RELEASE file

RELEASE.md

+# How to release a new version
+
+Note: You need the release-key to bundle a release that can be uploaded to Google Play Store.
+
+Run the release wizard that takes you through all the steps necessary to prepare a new release:
+
+```
+./tools/release-wizard.js
+```
+This will trigger the following:
+* update version number
+* create a new tag for the release
+* update the changelog
+* make a release commit
+
+To then bundle a release run the following command on your branch:
+
+```
+cd android && ./gradlew bundleRelease
+```
+
+This command creates an `app.aab` file in the folder `/android/app/build/outputs/bundle/release`.
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