Add description of how surface was generated

[jennyfothergill]

TODO comments from mbuild/lib/surfaces/amorphous_silica_surface.py

11-        if surface_roughness == 1.0:
12:            # TODO: description of how this surface was generated/citation
13-            mb.load('amorphous_silica_sr1.0.pdb', compound=self,
14-                    relative_to_module=self.__module__)

related to #872

[CalCraven]

We would need someone with that information here. @chrisiacovella do you have that info anywhere?