Start a guide regarding how to backport code to Bitcoin-ABC

Summary:
So that we don't end up with a variety of different styles of backporting,
this is an attempt at a rough guide for backporting.

Test Plan: Read the document

Reviewers: deadalnix, jasonbcox, matiu, #bitcoin_abc

Reviewed By: jasonbcox, matiu, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D980