Added migrating to 2.x docs

In a similar vein to the migrating to 1.x docs, this section details changes that can break existing code in order to ease the migration to 2.x. I didn't talk about any of the added APIs since those don't really break existing code. Compared with the last major release, there's not much to say.

This is largely based on @Lukasa's [blog post](http://lukasa.co.uk/2013/09/Requests_20/) and the [changelog](https://github.com/kennethreitz/requests/blob/master/HISTORY.rst). I did not know how folks feel about linking to a blog post from the docs so I didn't add the link.