Add What's New section to docs

[wholmgren]

A good pattern seems to be to make a separate file for each release.

Possibly helpful links

https://github.com/pydata/pandas/tree/master/doc/source

https://github.com/ipython/ipython/tree/master/docs/source

https://github.com/statsmodels/statsmodels/tree/master/docs/source