add conda forge install instructions

[raybellwaves]

add conda forge install instructions here https://kubernetes.dask.org/en/latest/#install (https://github.com/dask/dask-kubernetes/blob/master/doc/source/index.rst#install).

Could copy from dask-jobqueue https://jobqueue.dask.org/en/latest/install.html#installing

[jrbourbeau]

Adding conda install instructions would be a nice addition to the docs. Any interest in submitting a PR? (no obligation though)

[raybellwaves]

Sure