Adding README file for the wakeonlan plugin

plugins/wakeonlan/README

+This plugin provides a wrapper around the "wakeonlan" tool available from most
+distributions' package repositories, or from the following website:
+
+http://gsd.di.uminho.pt/jpo/software/wakeonlan/
+
+In order to use this wrapper, create the ~/.wakeonlan directory, and place in
+that directory one file for each device you would like to be able to wake. Give
+the file a name that describes the device, such as its hostname. Each file
+should contain a line with the mac address of the target device and the network
+broadcast address.
+
+For instance, there might be a file ~/.wakeonlan/leto with the following
+contents:
+
+00:11:22:33:44:55:66 192.168.0.255
+
+To wake that device, use the following command:
+
+# wake leto
+
+The available device names will be autocompleted, so:
+
+# wake <tab>
+
+...will suggest "leto", along with any other configuration files that were
+placed in the ~/.wakeonlan directory.
+
+For more information regarding the configuration file format, check the
+wakeonlan man page.