Release notes & version 3.0.1

It's ready except for #2197 which will contradict the deprecation policy.
I got lazy about this myself, but we prob oughta go with the `[Text][ref]` ... `[ref]: url` link style throughout.
So `[December 2014][3.0.1-milestone]` with `[3.0.1-milestone]: https://github.com/tomchristie/django-rest-framework/issues?q=milestone%3A%223.0.1+Release%22` at the bottom of the document.

Don't mind fixing all these up myself tho since it's my fault for getting lazy about it in previous versions.