Add simplest example straight in the docs + at the top of the main module's Haddock

[lucasdicioccio]

From Axman6's Reddit comment at https://www.reddit.com/r/haskell/comments/6tfurj/announce_first_release_of_http2client/ .

I may just not be familiar enough with HTTP2, but I can't actually tell how to make a GET request with this library. This seems kinda fundamental, so could you add some docs explaining how to do this?

[lucasdicioccio]

I've merged #21 , which contains a literate haskell example.