DOC: Contributing / isort (#1947)
DOC: Contributing / isort (#1947)

* flake8 only

without stepping on @jhamman's toes, flake8 takes 5.4s to run and so we
can probably cut to just running `flake8` rather than an explanation of
the diffs etc

* isort instructions

jhamman/xarray,markelg/xray,shoyer/xarray,shoyer/xarray,jhamman/xarray,markelg/xray,jhamman/xarray,chunweiyuan/xarray,xray/xray,shoyer/xray,pydata/xarray,jcmgray/xarray,pydata/xarray,jhamman/xray,pydata/xarray,markelg/xray