add upgrade guide

Author: tabacitu

readme.md

@@ -36,6 +36,16 @@ You can use elFinder in Backpack:
 - stand-alone, by accessing the ```/admin/elfinder``` route (see screenshot above);
 - inside the [```browse```](https://backpackforlaravel.com/docs/4.1/crud-fields#browse), [```browse_multiple```](https://backpackforlaravel.com/docs/4.1/crud-fields#browse_multiple) or [```ckeditor```](https://backpackforlaravel.com/docs/4.1/crud-fields#ckeditor) field types;
 
+## Upgrade
+
+To upgrade from v2 to v3 of this package:
+```bash
+# remove the published blade views
+rm -rf resources/views/vendor/elfinder
+
+# publish the new blade views
+php artisan backpack:filemanager:install
+```
 
 ## Security