Initial draft of HTTP streaming documentation

Problem

It's too difficult to learn HTTP streaming by reading the source, and
the test cases in EndToEndTests.scala are too alien looking to model
real world problems on.

Solution

Documentation and examples of usage.

Result

More people know about HTTP streaming and will be comfortable to try it
out.
While technically correct, I don't think "fixed length" is quite the right phrase here. Usually that implies it is fixed across *all* requests. (E.g. in protocols, we talk about 'fixed-width' fields)

Maybe instead say something like: ".. encountered so far are simple request-response services"

(Or, ideally something even better!)