Update plugins.rst

Added information about headers not present in soap enevelop in egress plugin

Author: hmol

docs/plugins.rst

@@ -27,7 +27,10 @@ Writing a plugin is really simple and best explained via an example.
 
 
 The plugin can implement two methods: ``ingress`` and ``egress``. Both methods
-should always return an envelop (lxml element) and the http headers.
+should always return an envelop (lxml element) and the http headers. The 
+envelope in the ``egress`` plugin will only contain the body of the soap message.
+This is important to remember if you want to inspect or do something 
+with the headers.
 
 To register this plugin you need to pass it to the client. Plugins are always
 executed sequentially.