[doc] document the procedure to backport from subtree projects

Summary: Add a section to backporting.md about how to backport commits from secp256k1, Electrum and Electron Cash.

Test Plan: Procedure tested in D15522 and other previous backports

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15523