doc: add squash guideline to pull-requests doc

Tell the contributor to generally not squash commits during the pull
request review process.

PR-URL: https://github.com/nodejs/node/pull/20413


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>