A first pass at What's New document - fixes 300

Fixes #300.

Note that is distinct from an auto-generated NEWS document; the "What's New" is a documented that should be written to point out the highlights and caveats of a release and is in part a marketing tool. NEWS will be a much more detailed low level "we fixed bugs X, Y, Z and added features A, B, C" kind of document.

@ferrantim @lukemarsden please comment.
Can we add something like "Everything!"  This is the first What's New document which is a little weird since everything is new.  It also makes it pretty clear why the limitations section is so large and scary.