Document mercurial and immutable history doctrines
Summary: Explains how to use the immutable history doctrine and mercurial.
Recommends "one idea is one commit".
Test Plan: Read documentation.
Reviewers: fratrik, Makinde, aran, jungejason, tuomaspelkonen, cpiro
Reviewed By: cpiro
CC: aran, cpiro, epriestley, ide
Differential Revision: 861