Add docs on downloading new QCFractal datasets (or including your own) and building new force fields

[jchodera]

I think we need more extensive documentation on how to build new force fields from QM datasets (either QCFractal or from somewhere else)?

[jarvist]

This would be super useful for us. We'd like to extend Espaloma to organic electronic materials (where we otherwise spend a lot of time patching OPLS or similar to include the stiffening in the ring systems due to the conjugation). But it's quite perplexing where to get started: there's the quick QC tutorial to do from raw text files (very useful), but then the technical leap to preparing data for QCFractal etc. for eventual inclusion in the main datastream is enormous.

We'd be happy to write some documentation as we figure this out, but we might need to consult some of the developers just to get it working!