R1:49205f977cca

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…