Improve documentation

We should potentially improve our documentation around some of the slightly "fuzzier" aspects of ShareDB. Some things that have come up recently are:

• Doc lifecycle - how to properly get, fetch, and destroy documents; should also point out gotchas like the fact that there's a caching layer on the connection, so if I do connection.get twice, I don't have two independent instances of Doc (eg if I wanted to subscribe on one, but just fetch on the other, it's non-obvious that these two documents are secretly the same thing); also you can't doc.destroy and then doc.fetch, etc.
• authentication - there's a short mention of a plugin in the README, but it doesn't address some of the weirder cases, like trying to authenticate rich-text (which doesn't have a logical place to store metadata on the document itself). We should also point out that things like ops should be secured as well as documents (which can be a bit non-obvious when coming from a more "traditional" data store)

In general, I kind of think we've out-grown the README, and it might be nice to throw together some sort of HTML documentation, with code snippets, etc., but obviously that'll be quite a lot of work.

Start with the README's existing authentication plugin section and the documentation around the Doc lifecycle, connection.get, fetch, destroy, and rich-text authentication. Define the documentation scope before choosing between an expanded README and HTML pages; done means the lifecycle, caching, authentication, and operation-security cases are explained with useful code snippets.