README: Add reference to FFIG
Per discussion with Ralf
See extended info here: https://github.com/RobotLocomotion/drake/issues/7889#issuecomment-752841757
Edited by Eric Cousineau
Per discussion with Ralf
See extended info here: https://github.com/RobotLocomotion/drake/issues/7889#issuecomment-752841757