Document release process and release guidelines

[BWibo]

• It is not documented how to create releases for this repository.

  • How to release a beta/release version?
  • How to make use of the helm chart releaser workflows?
  • PR and reviews are requried for new releases (maybe not for beta?)

• There are no guidelines specified for releases.

  • Proper semantic versioning (release version and chart version)
  • CHANGELOG is kept up to date
  • Chart documentation is in sync with the chart