DOC: document groupby.resample/rolling

[jreback]

examples are in the whatsnew: http://pandas.pydata.org/pandas-docs/stable/whatsnew.html#groupby-syntax-with-window-and-resample-operations

but I think a section in groupby.rst with some xrefs in timeseries.rst (resample) and computation.rst (rolling) would be helpful. I wrote this and can barely find it!

plus the API reference is missing these functions

[wandersoncferreira]

hi @jreback I want to start contributing to pandas project and I think this could be a good entry for me. I use this two method all day long.

Can you assign this to me? Thanks!