Add notice about maintenance mode (#1259)

Author: danoscarmike

README.md

@@ -2,6 +2,9 @@
 
 # Google APIs Client Library for PHP #
 
+## 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.
+
 ## Description ##
 The Google API Client Library enables you to work with Google APIs such as Google+, Drive, or YouTube on your server.