Decide guide policy for PRs that need to wait

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
1. 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.