Update Create Drupal User plugin's documentation

plugin/createdrupaluser/README.md

@@ -1,4 +1,28 @@
-Create Drupal user
-==================
+Create Drupal user plugin
+=========================
+
+This plugin creates a user on a Drupal portal when a user is registered in 
+Chamilo.
+
+This uses the Hook mechanism available to Chamilo plugins: when enabling this 
+plugin, the HookCreateDrupalUser is automatically added to the hooks stack, and
+the UserManager::create_user() method calls the HookCreateUser hook and notifies
+it, resulting in the plugin code to be executed.
+
+The Drupal portal settings must be configured in the configuration panel for the
+plugin. A SOAP call is then initiated inside the plugin code, that will use the
+Drupal's addUser() web service. See src/HookCreateDrupalUser.php for more info
+on the call parameters.
+
+After calling the web service and receiving a positive answer, Chamilo stores
+the remote (Drupal) user ID inside the extra_field_values table, as field name
+"drupal_user_id". This later serves for updates and other synchronisation
+purposes.
+
+Extending
+---------
+
+Other plugins could easily be created by copying this one and modifying the
+class names and web services to call. Simply review every variable coined with
+the "drupal" name and update according to your own portal.
 
-Create a user in Drupal website when a user is registered in Chamilo LMS