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