Add/update some documentation on branches and reviewable code

Summary:

• The most common workflow complaint I've seen recently is something like "how do I use Differential with a branch full of random code that me and several other developers all commit to"? There are some okay answers ("commandeer") but I think the best answer is "don't do that". Add a document explaining how development works at Facebook (and many other companies) without the use of feature branching, why it's better, and how you can lay the technical groundwork you need to to stop doing this.
• Add a general "smaller commits are better" and "your commit messsage should provide context" document.
• Minor updates to other stuff as my understanding of Mercurial has been refined.

Test Plan: Generated and read documentation.

Reviewers: btrahan, vrana, schrockn

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3025