added link to composer docs about autoloading

docs/guide/extensions.md

@@ -37,6 +37,8 @@ using the following syntax: e.g. `[[name()]]`, `[[name\space\MyClass::name()]]`.
     },
     ```
     
+    Details about autoloading configuration can be found in the [composer documentation](http://getcomposer.org/doc/04-schema.md#autoload).
+
 
 
 ### Namespace and package names