Add README.md

README.md

+This repository allows to run Rail Environment multi agent training with the RLLib Library.
+To start a grid search on some parameters, you can create a folder containing a config.gin file.
+Then, you can modify the config.gin file path at the end of the grid_search_train.py file.
+
+The results will be stored inside the folder, and the learning curves can be visualized in 
+tensorboard: 'tensorboard --logdir=/path/to/foler_containing_config_gin_file'.
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