Add a note about migrations to the UPGRADE file

Author: GSadee

UPGRADE-2.0.md

@@ -8,3 +8,10 @@
 
 1. The `sylius_invoicing_plugin_admin_order_invoices_partial` and `sylius_invoicing_plugin_shop_order_invoices_partial` partials 
    have been replaced by `Sylius\InvoicingPlugin\Twig\Component\Invoice\ListComponent` twig component.
+
+1. Doctrine migrations have been regenerated, meaning all previous migration files have been removed and their content 
+   is now in a single migration file. To apply the new migration and get rid of the old entries run migrations as usual:
+
+   ```bash
+       bin/console doctrine:migrations:migrate --no-interaction
+   ```