DOC: add a section on growing pandas ecosystem
Incresingly, python packages are being built on top of pandas.
Often, these solve problems pandas users need addressed
but which are a stretch for pandas' scope proper.
#2302, and [pandas-xlsxwriter-charts](http://pandas-xlsxwriter-charts.readthedocs.org/en/latest/chart_examples.html#scatter-chart), for example. vincent, yhat/ggplot, seaborn, etc'.

statsmodels too ofcourse, though it should be fairly well known.

Let's give them some exposure, encourage people to do their
own thing on top of pandas and help users locate existing solutions.

Any more mature(-ish) packages which deserve a mention?
