add doc about Local adapter directory deletion feature

doc/adapters/local.md

@@ -22,3 +22,17 @@ use Gaufrette\Adapter\Local as LocalAdapter;
 $adapter = new LocalAdapter('/var/media', true, 0750);
 $filesystem = new Filesystem($adapter);
 ```
+
+## Delete a directory
+
+Directory deletion is a feature only available on the `Local` adapter. It is
+not supported by the `FilesysteInterface` which aims to only deal with objects.
+
+Following the above statement, you have to explicitely retrieve the adapter
+in order to delete a directory :
+
+```php
+$filesystem->getAdapter()->delete($dirKey);
+```
+
+Note that you can't delete the root directory of the Local adapter.