tools/relnotes: automate CHANGELOG generation

[hyangah]

The most time consuming part in our release process is the CHANGELOG generation.

• Establish a convention to provide change log content. Some options:

  • require the authors of new features or fixes to update CHANGELOG as part of their CLs/PRs, if they are worthy of mention in release note
  • require the last comment in the fixed issue to include the details and let a tool collect the last comment.
  • require a special tag in the gerrit cl comment and let a tool collect the commit message or release note embedded in the commit message.

• Enhance tools/relnotes to utilize the convention

  • make sure the authors/contributors names are included correctly.
  • make sure the issues/cls/milestone links are included correctly.

• Update the release plan & instruction

cc @golang/tools-team