Add "how to use plugins" page

doc/plugin/use.rst

@@ -0,0 +1,83 @@
+.. _plugin-use:
+
+=====================
+ Using munin plugins
+=====================
+
+.. index::
+   pair: plugin; installing
+
+Installing
+==========
+
+The default plugin directory is /etc/munin/plugins/.
+
+To install a plugin, place it in the plugin directory, and make it
+executable.
+
+You can also place the plugin elsewhere, and install a symbolic link
+in the plugin directory. All the plugins provided with munin are
+installed in this way.
+
+.. index::
+   pair: plugin; configuration
+
+Configuring
+===========
+
+The plugin configuration directory is /etc/munin/plugin-conf.d/. The
+syntax is:
+
+user <username>
+  The user the plugin will run as.
+
+  Default: munin
+
+group <groupname>
+  The group the plugin will run as
+
+  Default: munin
+
+env.variablename <variable content>
+  Defines and exports an environment variable called "variablename"
+  with the content set to <variable content>.
+
+  There is no need to quote the variable content.
+
+.. note::
+
+   When configuring a munin plugin, add the least amount of extra
+   privileges needed to run the plugin. For instance, do not run a
+   plugin with "user root" to read syslogs, when it may be sufficient
+   to set "group adm" instead.
+
+Example:
+
+.. index::
+   triple: example; plugin; configuration
+
+::
+
+   [pluginname]
+   user             username
+   group            groupname
+   env.variablename some content for the variable
+   env.critical     92
+   env.warning      95
+
+Plugin configuration is optional.
+
+.. index::
+   pair: plugin; testing
+
+Testing
+=======
+
+To test if the plugin works when executed by munin, you can use the
+:ref:`munin-run` command.
+
+.. code-block:: bash
+
+   # munin-run myplugin config
+
+   # munin-run myplugin