added attributes for worlds

perls2/worlds/README.md

@@ -1 +1,26 @@
 # World
+
+A World encompasses the simulation/reality of the experimental setup. Worlds contain specifications for the physics of simulation (gravity, timesteps, collisions.) Worlds contain arenas and interfaces to robots, objects and sensors. Worlds do **not** typically contain information relevant to the task(i.e. reward functions, observations, actions, terminations.) Environments take the state of the World to produce an observation, and apply actions to the world at each step. 
+
+
+## Hierarchy: 
+
+## Attributes:
+-**config**: dict
+            Config files contain parameters to create an arena, robot interface,
+            sensor interface and object interface. They also contain specs for
+            learning, simulation and experiment setup.
+
+-**arena**: Arena
+            Manages the sim by loading models (in both sim/real envs) and for
+            simulations, randomizing objects and sensors parameters
+
+-**robot_interface**: RobotInterface
+            Communicates with robots and executes robot commands. Robot Interfaces are also typically constructed by the robot_factory.
+
+-**sensor_interface**: SensorInterface
+            Retrieves sensor info and executes changes to params    
+
+-**object_interface**: ObjectInterface
+            Only for Sim environments-- retrieves object info and excecutes
+            changes to params