Document `fix:` and other commit messages that allow CI to automatically publish a new version of gitlab-ui

Document fix: and other commit messages that allow CI to automatically publish a new version of gitlab-ui

The following discussion from !272 (merged) should be addressed:

• @filipa started a discussion: (+1 comment)

  @lauraMon LGTM, thanks! I think we need to start the commit message with fix: in order for CI to pick the job and automatically publish a new version 🤔

  I can't find docs for it though 🤔 @iamphill do you know?