Improve docs around which lifecycle methods are safe for side effects

[bvaughn]

This issue was originally reported by @jedwards1211 via facebook/react/issues/9693

Outcome of discussion on this issue: We should better document that you're not expected to cause side effects that synchronously update state in certain lifecycle methods. Here's a cheatsheet that shows which methods are safe and which aren't.

[bvaughn]

Marking this in-progress since @jedwards1211 has submitted a PR

[bvaughn]

Closed by #88