ARTEMIS-898 - Adding documentation for Plugin support

Started a new chapter in the user manual for plugin support

docs/user-manual/en/SUMMARY.md

@@ -37,6 +37,7 @@
 * [Extra Acknowledge Modes](pre-acknowledge.md)
 * [Management](management.md)
 * [Security](security.md)
+* [Broker Plugins](broker-plugins.md)
 * [Resource Limits](resource-limits.md)
 * [The JMS Bridge](jms-bridge.md)
 * [Client Reconnection and Session Reattachment](client-reconnection.md)

docs/user-manual/en/broker-plugins.md

@@ -0,0 +1,46 @@
+# Apache ActiveMQ Artemis Plugin Support
+
+Apache ActiveMQ Artemis is designed to allow extra functionality to be added by
+creating a plugin. Multiple plugins can be registered at the same time and they will be chained
+together and executed in the order they are registered.  (i.e. the first plugin registered 
+is always executed first).
+
+Creating a plugin is very simple. It requires implementing the [`ActiveMQServerPlugin`](../../../artemis-server/src/main/java/org/apache/activemq/artemis/core/server/plugin/ActiveMQServerPlugin.java)
+interface, making sure the plugin is on the classpath, and registering it with the broker.  Only the methods that you want to add behavior for need to be implemented as all of the interface methods are default methods.
+
+## Registering a Plugin
+
+To register a plugin with by XML you need to add the `broker-plugins` element.
+
+`broker.xml` in your classpath:
+
+```xml
+<configuration ...>
+
+...
+    <broker-plugins>
+        <broker-plugin class-name="some.plugin.UserPlugin" />
+    </broker-plugins>
+...
+
+</configuration>
+```
+
+## Registering a Plugin Programmatically
+
+For registering a plugin programmatically you need to call the
+registerBrokerPlugin() method and pass in a new instance of your plugin.  In the example below
+assuming your plugin is called `UserPlugin`, registering it looks like the following:
+
+
+``` java
+
+...
+
+Configuration config = new ConfigurationImpl();
+...
+
+config.registerBrokerPlugin(new UserPlugin());
+```
+
+