Merge pull request #29 from slogan621/master

add requirement for pypubsub, and small blurb abount pip3

Author: driscollis

chapter9_nasa_image_downloader/README

@@ -8,4 +8,13 @@ You also need requests:
 
 https://pypi.org/project/requests/
 
-pip install requests
+pip install requests
+
+Finally, you will need pubsub:
+
+https://pypi.org/project/PyPubSub
+
+pip install pypubsub
+
+In python 3.x environments (like Ubuntu 20.04), use 'pip3' instead of 'pip' 
+in the above.