[Documentation] First commit to the Glossary (#452)

docs/Glossary.md

 # ML-Agents Glossary
 
-**Work In Progress**
+ * **Academy** - Unity Component which controls timing, reset, and 
+ training/inference settings of the environment. 
+ * **Action** - The carrying-out of a decision on the part of an 
+ agent within the environment.
+ * **Agent** - Unity Component which produces observations and 
+ takes actions in the environment. Agents actions are determined 
+ by decisions produced by a linked Brain.
+ * **Brain** - Unity Component which makes decisions for the agents 
+ linked to it.
+ * **Decision** - The specification produced by a Brain for an action 
+ to be carried out given an observation. 
+ * **Editor** - The Unity Editor, which may include any pane 
+ (e.g. Hierarchy, Scene, Inspector). 
+ * **Environment** - The Unity scene which contains Agents, Academy, 
+ and Brains.
+ * **FixedUpdate** - Unity method called each time the the game engine 
+ is stepped. ML-Agents logic should be placed here.
+ * **Frame** - An instance of rendering the main camera for the 
+ display. Corresponds to each `Update` call of the game engine.
+ * **Observation** - Partial information describing the state of the 
+ environment available to a given agent. (e.g. Vector, Visual, Text)
+ * **Policy** - Function for producing decisions from observations.
+ * **Reward** - Signal provided at every step used to indicate 
+ desirability of an agent’s action within the current state 
+ of the environment.
+ * **State** - The underlying properties of the environment 
+ (including all agents within it) at a given time.
+ * **Step** - Corresponds to each `FixedUpdate` call of the game engine. 
+ Is the smallest atomic change to the state possible.
+ * **Update** - Unity function called each time a frame is rendered. 
+ ML-Agents logic should not be placed here.
+ * **External Coordinator** - ML-Agents class responsible for 
+ communication with outside processes (in this case, the Python API).
+ * **Trainer** - Python class which is responsible for training a given 
+ external brain. Contains TensorFlow graph which makes decisions 
+ for external brain.