Consolidation of contributing guides

I had a user ask me today about contributing guidelines on `dask-kubernetes`. That project doesn't currently have a `CONTRIBUTING.md` file and so I pointed them to https://docs.dask.org/en/latest/develop.html#contributing-to-code.

Having a look through the various projects in the Dask org there isn't much consistency when it comes to contributing guidelines. Some projects do not have a contributing file (dask, dask-kubernetes, dask-yarn, dask-jobqueue). Some have a minimal file which points to the Dask docs link above (distributed). And some have their own parallel set of contributing guidelines (dask-ml).

I think it would be good to go through the projects without a `CONTRIBUTING.md` file and add a minimal one which points to the main Dask docs. We may also want to consider whether we should unite the dask-ml and dask-docs ones into a single one, or whether the projects are different enough that they require two sets of documentation.

Thoughts?