Add notification of maintenance mode

Author: danoscarmike

README.md

@@ -1,3 +1,6 @@
+# Library maintenance
+This client library is supported but in maintenance mode only.  We are fixing necessary bugs and adding essential features to ensure this library continues to meet your needs for accessing Google APIs.  Non-critical issues will be closed.  Any issue may be reopened if it is causing ongoing problems.
+
 # About
 This is the Python client library for Google's discovery based APIs. To get started, please see the [full documentation for this library](http://google.github.io/google-api-python-client). Additionally, [dynamically generated documentation](http://api-python-client-doc.appspot.com/) is available for all of the APIs supported by this library.