Document use of checklists and draft PRs in developer docs

[jtniehof]

We use the PR checklist as a guide for things to check in a pull request, but this does mean it's shown on the PR list as (e.g.) "7/8 tasks completed", implying the PR is not done or not complete. Another way of indicating not complete is using a draft PR instead of ready-to-merge. We should document in the dev docs (probably the working with PRs thing) what standards we want: do we want the checkmarks always checked (and if not applicable, put "N/A" and check it off, or maybe delete it?) or just assume any PR not in draft state is mergeable, regardless of checkmarks?

Closure condition

This issue should be closed when: dev docs updated as described above.

[jtniehof]

Examples of how people have gone about this in SpacePy: https://github.com/spacepy/spacepy/pulls?q=is%3Apr+is%3Aclosed