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.
+1 on removing orchestration.  I think we still need the word tool though, or something like it, since Flocker isn't a cluster itself, but a tool to manage clusters.  So maybe "Flocker is a data volume manager and multi-host Docker cluster management tool".  Also, I added "data" to "data volume manager" because that is how Docker references to volumes (https://docs.docker.com/userguide/dockervolumes/)