Document how to use book multiple cores for a single task

This is to clarify one of the outcome of #181.

Several people have asked how they can use dask-jobqueue to submit multi-threaded tasks. There are two answers currently:

- we can use `resource_spec` or specific scheduler equivalent kwarg to book more cores than nprocs*nthreads started with dask-worker.
- or we can use `resources` functionnality, which allows to submit both multi-threaded or single threaded tasks on the same cluster.

This needs to be documented.

@lesteve made up some examples: see https://github.com/dask/dask-jobqueue/issues/181#issuecomment-449341161.