Update README to include forcefield entry point details and/or instructions

[rmatsum836]

Describe the behavior you would like added to Foyer
Since we've established entry points for external force fields in Foyer, it would be a good idea to update the README and other docs to include the details of this procedure.

Additionally it might be a good idea to update the example forcefields with those that have entry points set up (perfluoroethers, GAFF, etc.)