Document Formatters

[fselmo]

What was wrong?

We are missing good documentation on our formatters. We should add documentation explaining how formatters work, what each of the default formatters do, and how to customize them if possible.

Since some of the middlewares moved into the formatters, this is especially important for a user's understanding of where and why certain formatting is happening.

[kclowes]

I wonder if it makes sense to pull @wolovim's blog post about JSON-RPC round trips into the docs, either via a link or as more of a guide.

[fselmo]

I think, in general, a section with external links to snakecharmer blog would be great. And having the formatters be documented more thoroughly in the docs in an appropriate section and perhaps linking out to that post for context on usage.

[kclowes]

@fselmo do you think this still needs to be done or did it get finished with your new middleware features?