Improve crafting release notes for new plugin releases

[sschuberth]

Currently, creating release notes for a new plugin release requires creating a dedicated file for that version, with a simple (yet undocumented) plain text format. Instead, it would be nicer if there was a single release notes file with all notes for all (past and current) releases present in the repository, so that

1. all release notes are visible at a single glance
2. the expected format is more clear by providing examples

Additionally, it would be nice if notes were already published to GitHub releases.

See #37 for context.

[sschuberth]

Maybe the same mechanism as for https://github.com/gradle/github-dependency-graph-gradle-plugin/releases could be used.