Document release process expectations

[joshuagl]

When looking at the POUF for the reference implementation I realised that we should probably make an effort as part of the release process to ensure it's up to date.

Furthermore we should document the release process and a pre-release checklist of tasks that should be completed before tagging a release.

Possible items for a pre-release checklist:

• Ensure the TUF version number written to metdata files is correct and up-to-date
• Ensure that POUF 1 is up-to-date
• Ensure the CHANGELOG lists major changes
• Bump the version number

Possible items for the release process:

• tag the release with a signed tag
• generate a Python wheel and upload to PyPI
• generate archives for inclusion in the GitHub release
• Add release information to GitHub with generated archives

[joshuagl]

Documented in #1094