Add a PR checklist item for "I have read the contributing guide..." as a catch-all

[dbeatty10]

resolves #904

This is a:

• under the hood change

Problem

Our PR template is pretty long, which makes it onerous to use. Many items are already covered in our contribution guidelines in their own section.

Solution

Replace (4) checklist items with (1) checklist item that says:

• I have read the contributing guide and understand what's expected of me

Checklist

• This code is associated with an Issue which has been triaged and accepted for development.
• I have read the contributing guide and understand what's expected of me
• I have updated the README.md (if applicable)
• I have added tests & descriptions to my models (and macros if applicable)