Document usage

Not sure why I haven't done it but provide examples on how to use in a README or even the top of the script.

See how the commits in one of our existing CHANGELOG.mds are formatted.
https://gitlab.codeopensrc.com/os/react-template/-/blob/master/CHANGELOG.md

Notable things are

• Show example of a credit trailer
  • Ex: Credit: First Last <email@domain.com>
  • Not sure if Last or <email> works though. Maybe name only works, not sure, double check
  • Also links to Merge Request created by author, does this by finding the merge requests that match SHA
• Show example of the issue trailer
  • Ex: Issue: #424 or Closes: #245 or Addresses: #532
  • List of issue trailers
  • ISSUE_TRAILER_KEYS="key=Issue,key=Closes,key=Close,key=Fix,key=Fixes,key=Related,key=Addresses"
• Show different categories
  • Ex: Changelog: massive or Changelog: added or Changelog: fixed
  • List of category trailers
  • CATEGORY_ORDER=( Massive Added Fixed Deprecated Removed Changed Security Performance Other )
• Combine trailers
  • Ex:

    • Changelog: added
      Credit: KC
      Related: #634