Clarify expectations for PR titles

[adamltyson]

When external collaborators contribute to our repos, there is a variety of styles when it comes to PR titles. We should clarify:

• PR titles should clearly explain the purpose of the PR
• PR titles should lean more towards "why", not "how". The "how" is usually contained within the code.
• We use squash merge, so PR titles should explain the entire PR
• We do not use any standards such as Conventional Commits, so PR titles should not include this metadata.