doc: add upgrade guide for 2.0 version

Author: Maxime Leclercq

UPGRADE-2.0.md

@@ -0,0 +1,6 @@
+# UPGRADE FROM `1.3` TO `2.0`
+
+- Support of Sylius Plus RBAC is momentarily removed.
+- If you've customized the admin form, use twig hooks to add your fields.
+  (see [Twig Hooks config](src/Resources/config/sylius/twig_hooks.yaml)).
+- Template path has changed from `@MonsieurBizSyliusHomepagePlugin/Homepage/index.html.twig` to `@MonsieurBizSyliusHomepagePlugin/shop/homepage/index.html.twig`, particularly in the `monsieurbiz_sylius_homepage_homepage` route.