Add discussion of git-format-patch to Phabricator.html

Summary: There is a discussion of git-format-patch in GettingStarted guide, but no mention of it in the Phabricator.html page.

Reviewers: jyknight, delcypher

Reviewed By: delcypher

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69323