License

I noticed today that this repo doesn't have a license.

As a Dask project it should probably have the New BSD license. However at one point we merged it into [helm/charts](https://github.com/helm/charts) which is Apache 2.0 and we accepted contributions during that time. We then split off again as instructed by the helm maintainers.

As the Apache 2.0 license is slightly less permissive ([it is more prescriptive about including the license and other info](https://softwareengineering.stackexchange.com/questions/40561/is-bsd-license-compatible-with-apache)) I expect we must continue to use that license going forward.

What do folks think about me raising a PR to add the Apache license?