Document release process

[lann]

Probably won't be much more than proposed here for 0.2.0, but getting it written down in the repo would be good:

• Cut a release-0.2 branch from main
• Update main Cargo.toml version = "0.3.0-dev"
• In the release-0.2 branch:
  • Finalize 0.2.0 release:
    • Feature freeze / bug fix period (if desired)
    • Update changelog (if we want one in-tree)
  • PR/merge Cargo.toml version = "0.2.0"
  • Tag that commit as v0.2.0