add reference to stevenpjg's DDPG implementation

Author: jimkon

.gitignore

@@ -0,0 +1,10 @@
+
+# build dirs
+/__pycache__
+
+# .py files
+clipboard.py
+ou_noise.py
+
+# results
+results

README.md

@@ -1,2 +1,4 @@
 # Deep-Reinforcement-Learning-in-Large-Discrete-Action-Spaces
 Implementation of the algorithm in Python 3, TensorFlow and OpenAI Gym
+
+This paper introduces Wolpertinger training algorithm that extends the Deep Deterministic Policy Gradient training algorithm introduced in [this](https://arxiv.org/abs/1509.02971) paper. I extended stevenpjg's implementation of DDPG algorithm found [here](https://github.com/stevenpjg/ddpg-aigym) licensed under the MIT license.