[Docs] Add v6 upgrade instructions

docs/upgrading.md

@@ -6,7 +6,7 @@ weight: 6
 ALL upgrades of this package should follow these steps:
 
 1. Upgrading between major versions of this package always require the usual Composer steps:
-   > 1. Update your `composer.json` to specify the new major version, such as `^5.0`
+   > 1. Update your `composer.json` to specify the new major version, such as `^6.0`
    > 2. Then run `composer update`. 
 
 2. Compare the `migration` file stubs in the NEW version of this package against the migrations you've already run inside your app. If necessary, create a new migration (by hand) to apply any new changes.
@@ -17,6 +17,12 @@ ALL upgrades of this package should follow these steps:
 
 5. Apply any version-specific special updates as outlined below...
 
+### Upgrading to v6
+If you have overridden the `getPermissionClass()` or `getRoleClass()` methods or have custom Models, you will need to revisit those customizations. See PR #2368 for details. 
+eg: if you have a custom model you will need to make changes, including accessing the model using `$this->permissionClass::` syntax (eg: using `::` instead of `->`) in all the overridden methods that make use of the models.
+Be sure to compare your custom models with originals to see what else may have changed.
+
+Also note that the migrations have been updated to anonymous-class syntax that was introduced in Laravel 8. You may optionally update your original migration files accordingly. 
 
 ### Upgrading from v1 to v2
 If you're upgrading from v1 to v2, there's no built-in automatic migration/conversion of your data to the new structure.