Add a release guide/checklist/whatever

[triallax]

It's very easy to mess up creating a release, as we just did with v2.0.1 (we forgot to update the app version in pubspec.yaml and in-app). We should have a checklist or something like that for releasing an update. Even better would be automating some/all of the steps.

[triallax]

@zer0fl0w please don't make comments on unrelated issues.

To answer your question, we stopped uploading APKs until we can figure out how to handle signing the APKs, which we never did properly. You can use F-Droid instead, or wait until the next update in case we start uploading APKs again, as the new version contains only minor changes.