doc: note that PR-URL should be first metadata
Checklist
-
documentation is changed or added -
the commit message follows commit guidelines
Affected core subsystem(s)
doc
Description of change
Convention has coalesced around putting the PR-URL as the first metadata item in pull requests, so much so that collaborators are now flagging other collaborators' commit messages when they don't do that. So let's make it official and document that the PR-URL should be first.
Refs: https://github.com/nodejs/build/issues/325 Refs: https://github.com/nodejs/node/pull/6385