Add note on draft PRs in contributing document

[rjyounes]

Add note that reviews should not be requested until the PR is out of draft state.

[uscholdm]

If we do that, then we need to say what to do when a PR has a possible road block that needs discussion before a merge is ready. Someone needs to review the work to determine how to proceed. For example: #1077

[rjyounes]

Yes, this PR is what prompted me to add the issue. :)

OK, maybe not such a big deal that it needs to be documented. Closing.