doc: document use of the Squash and merge button
The Create a Merge Commit
and Rebase and Merge
buttons are not
currently suitable for merging Pull Requests, and are thus disabled.
Squashing is okay, so we should document its use.
Fixes: https://github.com/nodejs/node/issues/11674
Checklist
-
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc
cc/ @seishun @MylesBorins @evanlucas @mscdex @lpinca