Add the 'umbrella' DOI to the README.rst

See http://help.zenodo.org/#versioning

FIX Whatsnew page in docs

v0.3.3

A release for Python 3.6 compatibility

v0.3.2

- Bugfix release to patch over bug in scipy 0.18.0 cKDTree.

Merge pull request soft-matter#375 from caspervdw/zenodo

DOC New zenodo badge and doc reference

REL v0.3.1rc2

Merge pull request soft-matter#368 from caspervdw/release-notes

DOC Release notes for v0.3.1

REL: Tag release candidate v0.3.0rc1

BLD: Simplify setup. Require setuptools.