Added a note about gitian builds on local changes

Summary:
Tried doing a gitian build with local changes applied and realized I didn't know how.

Co-authored-by: Shammah Chancellor <shammah.chancellor@bitcoinabc.org>

Test Plan: Successfully ran a gitian build using local changes.

Reviewers: schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1416