Merge pull request #451 from openforcefield/link-ff-repos

Add section to README pointing to OFF FFs

Author: j-wags

README.md

@@ -29,6 +29,15 @@ Detailed installation instructions can be found [here](https://open-forcefield-t
 
 If you need to install via source, see the build and run package requirements listed in the [development conda recipe](https://github.com/openforcefield/openforcefield/blob/master/devtools/conda-recipe/meta.yaml).
 
+## Force Fields
+
+Two major force field development efforts have been undertaken by the Initiative, with results hosted in separate repositories. 
+
+* The [Open Force Fields repository](https://github.com/openforcefield/openforcefields/), which features the [Parsley force field line](https://openforcefield.org/news/introducing-openforcefield-1.0/). This is the Open Force Field Initiative's first effort toward building a _new_ force field. The initial parameters are taken from smirnoff99Frosst, but software and data produced by the Initiative's efforts are being used to refit parameter values and add new SMIRKS-based parameters. 
+* The [smirnoff99Frosst repository](https://github.com/openforcefield/smirnoff99Frosst/), which is descended from AMBER's parm99 forcefield as well as Merck-Frosst's parm@frosst. This line of force fields does not aim to alter parameter values, but is instead a test of accurately converting an atom type-based force field to the SMIRNOFF format. 
+
+Force fields from both of these packages are available in their respective GitHub repositories and also as conda packages. Tables detailing the individual file names/versions within these force field lines are in the README of each repository. By default, installing the Open Force Field toolkit using `conda` or the single-file toolkit installers will also install these conda packages. A plugin architecture is provided for other force field developers to produce python/conda packages that can be imported by the Open Force Field Toolkit as well. 
+
 # Toolkit features
 
 ## The SMIRKS Native Open Force Field (SMIRNOFF) format