update readme (#344)

examples/DDPG/README.md

 ## Reproduce DDPG with PARL
 Based on PARL, the DDPG algorithm of deep reinforcement learning has been reproduced, reaching the same level of indicators as the paper in Atari benchmarks.
 
-+ DDPG in
+> DDPG in
 [Continuous control with deep reinforcement learning](https://arxiv.org/abs/1509.02971)
 
 ### Mujoco games introduction

examples/DQN/README.md

 ## Reproduce DQN with PARL
 Based on PARL, we provide a simple demonstration of DQN.
 
-+ DQN in
+> DQN in
 [Human-level Control Through Deep Reinforcement Learning](http://www.nature.com/nature/journal/v518/n7540/full/nature14236.html)
 
 ### Result

examples/DQN_variant/README.md

 ## Reproduce DQN with PARL
 Based on PARL, the DQN algorithm of deep reinforcement learning has been reproduced, reaching the same level of indicators as the paper in Atari benchmarks.
 
-+ DQN in
-[Human-level Control Through Deep Reinforcement Learning](http://www.nature.com/nature/journal/v518/n7540/full/nature14236.html)
++ Papers: 
+
+> DQN in [Human-level Control Through Deep Reinforcement Learning](http://www.nature.com/nature/journal/v518/n7540/full/nature14236.html)
+> DDQN in [Deep Reinforcement Learning with Double Q-learning](https://arxiv.org/abs/1509.06461)
+> Dueling DQN in [Dueling Network Architectures for Deep Reinforcement Learning](https://arxiv.org/abs/1511.06581)
 
 ### Atari games introduction
 Please see [here](https://gym.openai.com/envs/#atari) to know more about Atari games.

examples/IMPALA/README.md

 ## Reproduce IMPALA with PARL
 Based on PARL, the IMPALA algorithm of deep reinforcement learning is reproduced, and the same level of indicators of the paper is reproduced in the classic Atari game.
 
-+ IMPALA in
+> IMPALA in
 [Impala: Scalable distributed deep-rl with importance weighted actor-learner architectures](https://arxiv.org/abs/1802.01561)
 
 ### Atari games introduction

examples/MADDPG/README.md

 ## Reproduce MADDPG with PARL
 Based on PARL, the MADDPG algorithm of deep reinforcement learning has been reproduced.
 
-+ paper: 
+> MADDPG in 
 [ Multi-Agent Actor-Critic for Mixed Cooperative-Competitive Environments](https://arxiv.org/abs/1706.02275)
 
 ### Multi-agent particle environment introduction