added documentation on the broadcast feature

docs/Readme.md

  * [Agents SDK Inspector Descriptions](Agents-Editor-Interface.md)
  * [Training on the Cloud with Amazon Web Services](Training-on-Amazon-Web-Service.md)
  * [Training with Curriculum Learning](curriculum.md)
+ * [How to use Broadcast](broadcast.md)
 
 ## Help
  * [Limitations & Common Issues](Limitations-&-Common-Issues.md)

docs/broadcast.md

+# Broadcast
+The Player, Heuristic and Internal brains have been updated to support broadcast. The broadcast feature allows you to collect data from your agents in python without controling them.  
+## How to use : Unity
+To turn it on in Unity, simply check the `Broadcast` box as shown bellow:
+
+![Broadcast](../images/broadcast.png)
+
+## How to use : Python 
+When you launch your Unity Environment from python, you can see what the agents connected to non-external brains are doing. When calling `step` or `reset` on your environment, you retrieve a dictionary from brain names to `BrainInfo` objects. Each `BrainInfo` the non-external brains set to broadcast.  
+Just like with an external brain, the `BrainInfo` object contains the fields for `observations`, `states`, `memories`,`rewards`, `local_done`, `agents` and `actions`. Note that `actions` corresponds to the actions that were taken by the agents at the previous step.  
+Note that when you do a `step` on the environment, you cannot provide actions for non-external brains. If there are no external brains in the scene, simply call `step()` with no arguments.  
+You can use the broadcast feature to collect data generated by Player, Heuristics or Internal brains game sessions. You can then use this data to train an agent in a supervised context.