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.
Maybe this should be in TOC page instead of here, with some text from README.