[Documentation] Added the On Demand Decision documentation. (#388)

* [Documentation] Added the On Demand Decision documentation.

* [Fixes] Corrected grammar mistakes

* [Documentation] Adding what kinds of games ODD is useful for

docs/ML-Agents-Overview.md

 However, this could also be helpful for the Heuristic and Internal Brains,
 particularly when debugging agent behaviors. You can learn more about using 
 the broadcasting feature [here](Feature-Broadcasting.md).
+
+* **On Demand Decision** - With ML-Agents it is possible to have agents 
+request decisions only when needed as opposed to requesting decisions at 
+every step. This enables training of turn based games, games where agents 
+must react to events or games where agents can take actions of variable 
+duration. Switching between decision taking at every step and 
+on-demand-decision is one button click away. You can learn more about the 
+on-demand-decision feature [here](Learning-Environment-On-Demand-Decision)

docs/Learning-Environment-On-Demand-Decision.md

+# On Demand Decision Making
+
+## Description
+On demand decision making allows agents to request decisions from their 
+brains only when needed instead of requesting decisions at a fixed 
+frequency. This is useful when the agents commit to an action for a 
+variable number of steps or when the agents cannot make decisions 
+at the same time. This typically the case for turn based games, games 
+where agents must react to events or games where agents can take 
+actions of variable duration.
+
+## How to use
+
+In the agent inspector, there is a checkbox called 
+`On Demand Decision`
+
+![Brain Inspector](images/ml-agents-ODD.png)
+
+ * If `On Demand Decision` is not checked, all the agents will 
+ request a new decision every `Decision Frequency` steps and 
+ perform an action every step. In the example above, 
+ `CollectObservations()` will be called every 5 steps and 
+ `AgentAct()` will be called at every step. This means that the 
+ agent will reuse the decision the brain has given it. 
+
+ * If `On Demand Decision` is checked, you are in charge of telling 
+ the agent when to request a decision and when to request an action. 
+ To do so, call the following methods on your agent component.
+   * `RequestDecision()` Call this method to signal the agent that it 
+   must collect its observations and ask the brain for a decision at 
+   the next step of the simulation. Note that when an agent requests 
+   a decision, it will also request an action automatically 
+   (This is to ensure that all decisions lead to an action during training)
+   * `RequestAction()` Call this method to signal the agent that 
+   it must reuse its previous action at the next step of the 
+   simulation. The Agent will not ask the brain for a new decision, 
+   it will just call `AgentAct()` with the same action.