add dask.imperative collections example
add dask.imperative collections example

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