Release 0.12

[smelc]

• Create & checkout release branch (release-<major>.<minor>).
• Fix minimal bazel version in the start script and
  the README, add to the CHANGELOG
  if it changed.
• Remove any feature that is still too experimental to go into a
  release, by cherry-picking reverts (or by manually deleting the
  feature).
  • Check the list below for PRs to remove.
• Write CHANGELOG by going through all pull requests since last
  release (https://github.com/tweag/rules_haskell/pulls?q=is%3Apr).
  • Add Highlights section for major improvements/changes
  • Added, Removed, Changed, Fixed as necessary
  • If relevant, add corresponding PRs to the changes
• Open a PR from the release branch, go through review and merge
  • If there is changes on the release branch that should not go to
    master, you create a second branch release-<major>.<minor>-master
    on master and cherry-pick all relevant commits from the release
    branch. Open a pull request with that branch, go through review
    and push changes back to the release branch.
• Verify that the final version of the release branch is green
• Create release tag (v<major>.<minor>) on the release branch and
  push
• Go to the release
  page
  • click on “Draft a new release”
  • name v<major>.<minor>
  • copy the changelog section into the description
  • release
• Push rules_haskell version in start script to new release tag,
  test it in a temporary directory, create PR against master
  • Publish start script and website by merging the commit into
    the release branch.
  • Check whether https://haskell.build/start has the newest start
    script (netlify has problems sometimes).
• Announce the new version (on Twitter)

[smelc]

Closing issue, final tweet is here: https://twitter.com/tweagio/status/1239874493604155398