doc: add guide for backporting prs

This guide should help answer questions for contributors
that are not familiar with the backport process.

PR-URL: https://github.com/nodejs/node/pull/11099


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>