Add comment about python runtime to Dockerfile (#662)

Need to link to source of Docker image

Author: jeffmendoza

endpoints/getting-started/Dockerfile.custom

@@ -1,3 +1,7 @@
+# The Google App Engine python runtime is Debian Jessie with Python installed
+# and various os-level packages to allow installation of popular Python
+# libraries. The source is on github at:
+#   https://github.com/GoogleCloudPlatform/python-docker
 FROM gcr.io/google_appengine/python
 
 RUN apt-get update && \