Write into CONTRIBUTING.md how to do a release

[ben18785]

[MichaelClerx]

Draft, please correct where wrong fergus :D

• Discuss w other devs
• Create branch
• Inspect changelog to determine next version number. First number: Almost never. Second number: Backwards incompatible changes. Third number: Bugfixes and/or new functionality only.
  • THere might be something about this in contributing.md already. Afaik we're doing multi-digit numbers (1.9 is followed by 1.10) and there is no significance to even/odd version numbers
• Add version number to changelog and remove empty sections
• Add version number to pints/version file
• Pass review
• Tag with "v" followed by version number, e.g. v0.20.1
• Merge: watch magic happen as pip updates? @fcooper8472 ?
• Update changelog to have new "Unreleased" section with all sections in correct order? Do we do this before the merge but after tagging? Separate PR? Not sure!