Add 'Raft in Practice' to README.

Per an e-mail conversation with @pvo, I added a section to the documentation called *Raft in Practice*.

@pvo Can you tell me if this helps to answer your question? Is there anything I'm missing?

@ongardie @kellabyte @xiangli-cmu @philips Can you guys give me a technical review? It's not very long.

Here's the [pretty printed version](https://github.com/goraft/raft/blob/docs/README.md#raft-in-practice).