message: added details about commit messages
diff: @@ -67,6 +67,8 @@ If a particular commit references another issue, please add the reference. For
  example: `refs #1234` or `fixes #4321`. Using the `fixes` or `closes` keywords
  will cause the corresponding issue to be closed when the pull request is merged.
  +Commit messages should never contain any `@` mentions.
  +
  Please refer to the [Git manual](https://git-scm.com/doc) for more information
  about Git.
  @@ -135,6 +137,10 @@ before it will be merged. The basic squashing workflow is shown below.
  # Save and quit.
  git push -f # (force push to GitHub)
  +Please update the resulting commit message if needed, it should read as a
  +coherent message. In most cases this means that you should not just list the
  +interim commits.
  +
  If you have problems with squashing (or other workflows with `git`), you can
  alternatively enable "Allow edits from maintainers" in the right GitHub
  sidebar and ask for help in the pull request.
  