Write blog post showing recommended, async-safe coding patterns

[bvaughn]

Follow up item for PR #12028. Relates to reactjs/rfcs/pull/6.

Show examples of how to accomplish common tasks in an async-friendly way, using the new component API.

Also show examples that illustrate pitfalls (eg setTimeout calls to setState).

[aweary]

This is done! 🎉

[bvaughn]

❤️

[blling]

Blog link? :)

[bvaughn]

https://reactjs.org/blog/2018/03/27/update-on-async-rendering.html