Repo cleanup prior to making public

[polarweasel]

Several things to do before we make this public:

• [x] License (I'm sorting this out shortly with legal)
• [x] Readme cleanup
• [x] Description cleanup
• [x] Release versioning scheme (SemVer, but do we need builds or -alpha etc suffixes?)
• [x] Further permissions/lockdown? (main is protected, PRs are required already)

[polarweasel]

I think the versioning scheme is OK as-is. Just use SemVer, and if we need an alpha build or whatever, just add the suffix.