Update README.

Added new description.

README.rst

@@ -17,6 +17,8 @@ Usage
                              [--port PORT]
                              [warning_threshold] [critical_threshold]
 
+    Check MySQL seconds behind master for Nagios-like monitoring.
+
     positional arguments:
       warning_threshold     Warning threshold (defaults to 60)
       critical_threshold    Critical threshold (defualts to 300)
@@ -30,6 +32,7 @@ Usage
       --host [HOST]         Login host
       --port PORT           Login port
 
+
 License
 -------