Generate changelog from makefile

[johanbrandhorst]

In the same vein as the grpc-gateway, simply generate the changelog. Reduces the amount of work involved in making a new release and general maintenance, at the cost of a neatly curated changelog. Opinions welcome.

Steps for making a new release (with this):

1. Create a new branch, generate CHANGELOG.md with

   $ NEW_RELEASE=<new_release> CHANGELOG_GITHUB_TOKEN=<token> make changelog
   

2. Merge CHANGELOG.md changes.
3. Tag new release at the new commit.

[codecov-io]

Codecov Report

Merging #266 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #266   +/-   ##
=======================================
  Coverage   73.31%   73.31%           
=======================================
  Files          42       42           
  Lines        1574     1574           
=======================================
  Hits         1154     1154           
  Misses        366      366           
  Partials       54       54

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[bwplotka]

Nice, I proposed removing it, let me know if this makes sense: https://github.com/grpc-ecosystem/go-grpc-middleware/pull/542

Rationale: GH releases allows nice generation of CHANGELOG straight there.

[bwplotka]

Alternative was merged, closing.