Update README.md

Author: dementrock

README.md

@@ -23,6 +23,8 @@ rllab only officially supports Python 3.5+. For an older snapshot of rllab sitti
 
 rllab comes with support for running reinforcement learning experiments on an EC2 cluster, and tools for visualizing the results. See the [documentation](https://rllab.readthedocs.io/en/latest/user/cluster.html) for details.
 
+The main modules use [Theano](http://deeplearning.net/software/theano/) as the underlying framework, and we have support for TensorFlow under [sandbox/rocky/tf](https://github.com/openai/rllab/tree/master/sandbox/rocky/tf).
+
 # Documentation
 
 Documentation is available online: [https://rllab.readthedocs.org/en/latest/](https://rllab.readthedocs.org/en/latest/).