Collision avoidance tutorial fix

docs/collision_avoidance_tutorial.md

@@ -181,6 +181,13 @@ class CollisionArmTable(RobotCollisionModel):
             limits['lift']=[None,table_height+safety_margin]        
         return limits
 ```
+Note that if the file is not created within the stretch_body directory then the following 
+command should be run in order to add the working directory to the PYTHONPATH env , This can also be added to our bashrc to permanently edit the path: 
+
+```bash
+>>$ export PYTHONPATH=$PYTHONPATH:/<path_to_modules>
+```
+
 
 Next we configure RobotCollision to use our CollisionArmTable model in `stretch_re1_user_yaml`:
 
@@ -194,6 +201,19 @@ collision_arm_table:
   py_class_name: 'CollisionArmTable'
   py_module_name: 'collision_arm_table'
 
+```
+However if we create our custom class within the 'stretch_body' directoy the yaml file should be configured as follows:
+
+```yaml
+robot_collision:
+  models:
+  - collision_arm_table
+
+collision_arm_table:
+  enabled: 1
+  py_class_name: 'CollisionArmTable'
+  py_module_name: 'stretch_body.collision_arm_table'
+
 ```
 
 Finally, test out the model by driving the arm and lift around using the XBox teleoperation tool: