Give a brief description on how to use plugins. Updates #11

Author: jcbrand

README.markdown

@@ -7,6 +7,21 @@ This is a collection of Strophe.js plugins created and maintained by the
 community. The homepage for this collection is
 https://github.com/strophe/strophejs-plugins .
 
+## How to use
+
+Strophe plugins extend the Strophe.Connection object by calling
+``Strophe.addConnectionPlugin``.
+
+This enables us to then access the plugin as an attribute on the connection
+object.
+
+For example, here the "roster" plugin is used:
+
+``
+    var connection = new Strophe.Connection(converse.bosh_service_url);
+    connection.roster.get(function () {});
+``
+
 ## Available Plugins
 
 ### XMPP Extensions