ci: add changelog update checker workflow

[galargh]

Resolves #267

This PR adds a CI workflow which checks whether the PR contains CHANGELOG.md modifications.

The check can be skipped by either adding [skip changelog] to the PR title or labelling the PR with skip/changelog. These instructions are included in the PR template and in the workflow failure message.

The checks succeeds if CHANGELOG.md was modified and fails otherwise.

Tested in galargh/.github#40

[codecov]

Codecov Report

Merging #398 (f479b6c) into main (cfcbdbc) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #398      +/-   ##
==========================================
+ Coverage   49.49%   49.53%   +0.04%     
==========================================
  Files         248      248              
  Lines       29838    29838              
==========================================
+ Hits        14769    14781      +12     
+ Misses      13640    13632       -8     
+ Partials     1429     1425       -4     

see 10 files with indirect coverage changes

[hacdias]

@galargh question: will we need to add [skip changelog] to release-related PRs?

[galargh]

@galargh question: will we need to add [skip changelog] to release-related PRs?

Good call! How about we trigger only on PRs that modify Go files (including go.mod/go.sum)? I think that should cover the cases where CHANGELOG update might be required.

[hacdias]

@galargh that still worries me: what about a cherry-picked release like #401, and a merge back PR like #403?

[galargh]

#401 modifies Go files so the changelog workflow run would be triggered. It also modifies CHANGELOG.md so the workflow run would have passed.

#403 doesn't modify Go files so with the changes from 71cac87 the changelog workflow wouldn't run at all.

[hacdias]

Okay, I think it sounds good. I'm happy to get this merged. We can always make any improvements if we find something.