Merge pull request #567 from grib0ed0v/patch-1

[UNIT 3] Update additional-readings.mdx

Author: simoninithomas

units/en/unit3/additional-readings.mdx

@@ -6,3 +6,4 @@ These are **optional readings** if you want to go deeper.
 - [Playing Atari with Deep Reinforcement Learning](https://arxiv.org/abs/1312.5602)
 - [Double Deep Q-Learning](https://papers.nips.cc/paper/2010/hash/091d584fced301b442654dd8c23b3fc9-Abstract.html)
 - [Prioritized Experience Replay](https://arxiv.org/abs/1511.05952)
+- [Dueling Deep Q-Learning](https://arxiv.org/abs/1511.06581)