D4291: contrib: import the relnotes script from the release-tools repo

[durin42 (Augie Fackler)]

durin42 added inline comments.

INLINE COMMENTS

> av6 wrote in relnotes:88-97
> Nit: this block looks too mundane to take 10 lines. The one below is even longer.

Leaving unfixed because I did the formatting with black, and I'd rather just use that than have style discussions.

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D4291

To: durin42, #hg-reviewers
Cc: av6, mjpieters, mercurial-devel