add link to gitter in docs
add link to gitter in docs

vikhyat/dask,blaze/dask,mrocklin/dask,ContinuumIO/dask,pombredanne/dask,chrisbarber/dask,ContinuumIO/dask,mrocklin/dask,dask/dask,gameduell/dask,jakirkham/dask,mraspaud/dask,jcrist/dask,mikegraham/dask,vikhyat/dask,jcrist/dask,cpcloud/dask,pombredanne/dask,jakirkham/dask,mraspaud/dask,blaze/dask,dask/dask,cowlicks/dask