Also:

- Clarify that PRs don't need to have associated tickets
- Clean up some language here and there

There are usually a few branches to choose from when picking a base-branch.
This breaks out a table to differentiate them.

If I disagree with using the "NFC" label, I'd like to be able to quickly
reference a link that explains the rationale and provides examples.

This makes it easier to reference these things from other pages.

https://wiki.civicrm.org/confluence/display/CRM/Pull-Request+Process

https://github.com/civicrm/civicrm-core/blob/master/.github/CONTRIBUTING.md

https://wiki.civicrm.org/confluence/display/CRMDOC/Git+Commit+Messages+for+CiviCRM

This isn't exactly an import. More of a re-write.