Added a reference to Monitor in Agents / Readme

- Also added references to Brain subpages

docs/Learning-Environment-Design-Agents.md

             provided the last time it requested a decision. 
 * `Decision Frequency` - The number of steps between decision requests. Not used if `On Demand Decision`, is true. 
 
+## Monitoring Agents
+
+We created a helpful `Monitor` class that enables visualizing variables within
+a Unity environment. While this was built for monitoring an Agent's value
+function throughout the training process, we imagine it can be more broadly
+useful. You can learn more [here](Feature-Monitor.md).
+
 ## Instantiating an Agent at Runtime
 
 To add an Agent to an environment at runtime, use the Unity `GameObject.Instantiate()` function. It is typically easiest to instantiate an agent from a [Prefab](https://docs.unity3d.com/Manual/Prefabs.html) (otherwise, you have to instantiate every GameObject and Component that make up your agent individually). In addition, you must assign a Brain instance to the new Agent and initialize it by calling its `AgentReset()` method. For example, the following function creates a new agent given a Prefab, Brain instance, location, and orientation:

docs/Readme.md

  * [Designing a Learning Environment](Learning-Environment-Design.md)
      * [Agents](Learning-Environment-Design-Agents.md)
      * [Academy](Learning-Environment-Design-Academy.md)
-     * [Brains](Learning-Environment-Design-Brains.md)
+     * [Brains](Learning-Environment-Design-Brains.md): [Player](Learning-Environment-Design-Player-Brains.md), [Heuristic](Learning-Environment-Design-Heuristic-Brains.md), [Internal & External](Learning-Environment-Design-External-Internal-Brains.md)
+ * [Using the Monitor](Feature-Monitor.md)
  * [TensorFlowSharp in Unity (Experimental)](Using-TensorFlow-Sharp-in-Unity.md)
  
 ## Training
  
 ## C# API and Components
  * Academy
- * Brain
- * CoreBrain
  * Decision
  * Monitor