docs: add documentation for publishing releases to f-droid (#2371)

README.md

@@ -53,3 +53,4 @@ Only for members of lichess team.
 
 1. Bump the pubspec.yaml version number. This can be in a PR making a change or a separate PR. Use semantic versioning to determine which part to increment. The version number after the + should also be incremented. For example 0.3.3+000303 with a patch should become 0.3.4+000304.
 2. Run workflow [Deploy to Play Store](https://github.com/lichess-org/mobile/actions/workflows/deploy_play_store.yml)
+3. [Publish on F-Droid](./docs/publish_fdroid.md)

docs/publish_fdroid.md

@@ -0,0 +1,20 @@
+# Publishing a new version to F-Droid
+
+There is a separate `fdroid` branch which uses unified push instead of firebase so that the app can be published on
+F-Droid. 
+To publish, first merge the new version tag into that branch.
+Then, ensure that the `flutter-version` file contains the exact version that the new Play Store release was built
+against (this is required for f-droid builds being reproducible).
+Finally, tag the release and push, this will automatically trigger a new version to be built by the F-Droid server.
+
+```bash
+git checkout fdroid
+git merge v<new-version>
+
+# If neccessary, update the flutter-version file
+
+# Any tag that ends with ".fdroid" will be picked up by the f-droid server as a new release
+git push
+git tag v<new-version>.fdroid
+git push --tags
+```