added comment regarding packages only required for example

Author: wumanu

environment.yml

@@ -11,7 +11,9 @@ dependencies:
   - gym==0.17.2
   - opencv-python==4.2.0.34
   - catkin-pkg==0.4.22
-  # the below packages are needed for stable-baselines
+  # the packages below are required to run the example (training with ppo) which
+  # we provide. if you do not wish to execute the example, you may safely remove
+  # them
   - tensorflow==1.15.0
   - pandas==0.24.2
   - joblib==0.16.0