Changelog update and check

[rjbou]

PR #4180 added a working changelog file master_changes.md and a github action to check that it is updated for each prs, if modifications are not whitelisted (infra/internal files, e.g. travis, readme, etc.).
It is a first step for minimal updates and checks.

(from #4180 (comment))

I dissociate the current changelog from the release one because they don't have the same purpose (neither the same format). The master_changes.md files contains all updates, so it contains almost all PRs (unless it's infra/CI ones). It is draft material to construct the release changelog and the release note.
The current changelog check/update is meant to evolve, to facilitate user pr process, e.g. for small prs, when PR title & changelog update are exactly the same, using a comment directive, an action to update automatically the PR.

On another hand, @dra27 began a same purpose project for ocaml repository, based on labels to dismiss some PRs check.

[dra27]

I think your master_changes.md check is working fine 🙂