Specify successful configurations for examples

[muupan]

Current examples don't specify in what configuration they work well, except newer ones (train_pcl_gym.py and train_reinforce_gym.py). Such instructions are important because users can easily confirm that the implementations actually work.

• [ ] ale/train_a3c_ale.py
• [ ] ale/train_acer_ale.py
• [ ] ale/train_dqn_ale.py
• [ ] ale/train_nsq_ale.py
• [x] gym/train_a3c_gym.py
• [ ] gym/train_acer_gym.py
• [ ] gym/train_ddpg_gym.py
• [x] gym/train_dqn_gym.py
• [x] gym/train_pcl_gym.py
• [x] gym/train_reinforce_gym.py

[prabhatnagarajan]

Shouldn't the default configuration work well?