Add steps to insall Draw.io extension

Guides/wiki.md

@@ -105,6 +105,17 @@ _todo_
 
 _todo_
 
+## Extensions
+
+### Draw.io
+
+[Extension:DrawioEditor](https://www.mediawiki.org/wiki/Extension:DrawioEditor)
+
+```
+cd /var/www/mediawiki/extensions/
+
+sudo git clone https://github.com/mgeb/mediawiki-drawio-editor DrawioEditor
+```
 ## Uninstall Mediawiki
 
 ### Disable site