Update README for Modules

src/Module/README.md

@@ -3,6 +3,21 @@
 The Module namespace contains the different modules of Friendica.
 Each module is loaded through the [`App`](https://github.com/friendica/friendica/blob/develop/src/App.php).
 
+There are mainly two types of modules:
+-	frontend modules to interact with users
+-	backend modules to interact with machine requests
+
+### Frontend modules
+
+This type of modules mainly needs a template, which are generally located at
+[view/templates/](https://github.com/friendica/friendica/tree/develop/view/templates).
+
+A frontend module should extend the [`BaseModule`](https://github.com/friendica/friendica/blob/develop/src/BaseModule.php), especially the `content()` method. 
+
+### Backend modules
+
+This type of modules mainly responds either with `XML` or with `JSON`. 
+
 Rules for Modules:
 - 	Named like the call (i.e. https://friendica.test/contact => `Contact`)
 -	Start with capitals and are **not** camelCased.