Decide guide policy for PRs that need to wait

[hillaryj]

I've seen two major schools of thought on how to indicate that a PR that GitHub thinks is ready to merge actually needs to wait. This status is different than draft PRs because the code is ready to be reviewed/merged but may need other PRs to be completed before merging can be done, i.e. for coordinating a new feature across several repositories.

1. Preface the title with [HOLD] or another keyword
2. Include a label i.e. DO NOT MERGE or with the feature i.e. HOLD: COOL FEATURE or (preferably for standardizing) use the blocked label

Since we want to standardize repository labels in https://github.com/cisagov/.github/issues/7, I'm recommending we adopt the title-preface method. We'll incorporate the results of discussion here into the team guide.

[dav3r]

I do like having a label for this, mainly because it catches my eye when I look at the list of PR reviewers (something I do just prior to merging), but I'm sure I could get used to the title-preface method also.

[hillaryj]

We could always do both - [HOLD] in the title and blocked as a label, perhaps?

[jsf9k]

I suggest the title thang, and possibly applying the blocked label as well. This is an out of the ordinary circumstance, so having two ways to flag such PRs will help prevent mistakes without being burdensome.