Added page outlining usage of plugins. (juju#1252)

* Added page outlining usage of plugins.

* Update juju-plugins.md

Author: evilnick

src/en/juju-plugins.md

@@ -0,0 +1,43 @@
+Title: Juju plugins
+
+# Juju plugins
+
+Plugins are commands which work with Juju, but which are not part of the core
+code. Some plugins are provided with your Juju install (e.g. 'juju-metadata') 
+because they contain functionality essential to some operations, others are
+just useful tools created by the community.
+
+## Community plugins
+
+There is a github project collecting all sorts of useful plugins created for
+the Juju ecosystem. You can check out further documentation on the plugins
+and how to install them [on the Github project page][plugin].
+
+## Running a plugin
+
+A Juju plugin is any executable code in your $PATH which begins 'juju-'. 
+Although you can run these independently of the Juju command line, Juju
+will also wrap these commands so they can be run within Juju. For example:
+
+```bash 
+juju-metadata generate
+```
+
+Can be executed within Juju by entering:
+
+```bash
+juju metadata generate
+```
+
+### Switches, flags and arguments
+
+There is potential confusion when a plugin accepts a switch or flag; Juju needs
+to be able to differentiate between options intended for Juju and ones intended
+for the plugin. In this case any options for the plugin should be preceeded by 
+' -- '. For example:
+
+```bash
+juju foo -- -bar --baz qux
+```
+
+[plugin]: https://github.com/juju/plugins#install "Juju plugins on Github"

src/navigation.tpl

@@ -54,6 +54,7 @@
                 <li><a href="juju-block.html">Restrict/block Juju commands</a></li>
                 <li><a href="juju-centos.html">CentOS support</a></li>
                 <li><a href="network-spaces.html">Network spaces</a></li>
+                <li><a href="juju-plugins.html">Juju plugins</a></li>
                 <li><a href="juju-misc.html">Miscellaneous</a></li>
             </ul>
         </li>