added DifferentiablePolicy to NAC docstring
added DifferentiablePolicy to NAC docstring

imanolarrieta/RL,BerkeleyAutomation/rlpy,MDPvis/rlpy,MDPvis/rlpy,rlpy/rlpy,silgon/rlpy,rlpy/rlpy,MDPvis/rlpy,imanolarrieta/RL,BerkeleyAutomation/rlpy,silgon/rlpy,silgon/rlpy,BerkeleyAutomation/rlpy,imanolarrieta/RL,rlpy/rlpy