Versionist could check that the CHANGELOG.md is in the correct format

[lurch]

Inspired by a discussion in balena-io-modules/open-balena-base#33

Given that we're mandating that all resin.io projects stick to a standard Changelog header-format (with the contents of the Changelog updated by versionist itself), perhaps it would make sense for versionist (or VersionBot, or SomeOtherBot*) to check that CHANGELOG.md (and perhaps other "standard files" too?) are following the expected conventions.

* hmmm, perhaps it'd be a RepoLintingBot ;-)

[lurch]

I guess the "default empty changelog" in #122 is a first step in this direction 😉