Document commit message linting

[apetro]

Related to #605 .

Document e.g. in CONTRIBUTING.md how one locally lints commit messages so that contributors can reduce surprise commit message linting failures at the CI layer by getting commit messages more right locally if able to.

I for one would gladly locally enjoy a commit hook that catches my non-conventional commit messages at the moment I try to commit them locally so I can fix them with the fastest available feedback loop. That's the moment I'm thinking about composing that commit message so at that moment removing a trailing . or fixing a capitalized Subject is cheap, easy, possibly even fun. Re-writing history to fix commit messages found wanting at the CI layer, that's less fun for me.

I'm not interested in forcing this workflow on others, it's just the workflow I'd like to have myself (and I'd like the documentation for this in the repo so that I and anyone else can achieve this workflow with minimal local making process up.)