add partitions section to dataframe docs
add partitions section to dataframe docs

cowlicks/dask,PhE/dask,blaze/dask,jayhetee/dask,jcrist/dask,clarkfitzg/dask,chrisbarber/dask,jcrist/dask,pombredanne/dask,pombredanne/dask,wiso/dask,vikhyat/dask,jakirkham/dask,vikhyat/dask,jayhetee/dask,dask/dask,jakirkham/dask,simudream/dask,dask/dask,ssanderson/dask,mrocklin/dask,PhE/dask,blaze/dask,ssanderson/dask,gameduell/dask,freeman-lab/dask,ContinuumIO/dask,mraspaud/dask,wiso/dask,simudream/dask,clarkfitzg/dask,cpcloud/dask,mrocklin/dask,ContinuumIO/dask,mraspaud/dask,freeman-lab/dask,mikegraham/dask