Update PR checklist for CHANGELOG.md

[TheAndrewJackson]

Purpose

Update the CHANGELOG.md instructions to be more clear.

Changes

• Add new checklist items for updating the CHANGELOG.md
• Add missing PR link from a previous merge

Checklist

• Update CHANGELOG.md file in appropriate sections
• Applicable documentation updated (guides, quickstart, postman collections, tutorial, fidesdemo, database diagram.
• If docs updated (select one):
  • documentation complete, or draft/outline provided (tag docs-team to complete/review on this branch)
  • documentation issue created (tag docs-team to complete issue separately)
• Good unit test/integration test coverage
• This PR contains a DB migration. If checked, the reviewer should confirm with the author that the down_revision correctly references the previous migration before merging
• The Run Unsafe PR Checks label has been applied, and checks have passed, if this PR touches any external services

Ticket

Fixes #535

[pattisdr]

Thanks for the improvements here @TheAndrewJackson