Add RELEASE.md and ensure function of automation for tagged releases

[consideRatio]

Going for binderhub 1.0.0, we should have clear release documentation as well

• PyPI trusted releases (see jupyterhub/team-compass#648)
  • release.yaml is trusted by PyPI
  • release.yaml is created etc
• RELEASE.md file (can be modelled from https://github.com/jupyterhub/jupyterhub-python-repo-template/blob/main/RELEASE.md)

[manics]

If we're publishing a separate npm client JS package (#1689) we'll need to include that here