docs/backend/core-services/permissionIntegrations: add migration guide

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

docs/backend-system/core-services/permissionIntegrations.md

@@ -12,3 +12,69 @@ This service allows your plugins to register new permissions, rules, and resourc
 For a deep dive into how to use the `permissionIntegrations` service, see the [permission guide for plugin authors](../../permissions/plugin-authors/01-setup.md).
 
 If all you want to do is add new custom permission rules to an existing plugin, you can instead refer to the [custom permission rules guide](../../permissions/custom-rules.md).
+
+## Migrating from `createPermissionIntegrationRouter`
+
+Before this service was introduced, plugins would use
+`createPermissionIntegrationRouter` to implement the same functionality. To
+migrate a plugin, locate the `createPermissionIntegrationRouter` call for your
+router and remove it, but copy all options that are passed to it, for example:
+
+```ts
+export async function createRouter() {
+  const router = Router();
+
+  /* highlight-remove-start */
+  const permissionIntegrationRouter = createPermissionIntegrationRouter({
+    resourceType: RESOURCE_TYPE_MY_RESOURCE,
+    permissions: [myResourcePermissions],
+    rules: [myResourceRule],
+  });
+
+  router.use(permissionIntegrationRouter);
+  /* highlight-remove-end */
+
+  // ...
+}
+```
+
+Next, add a dependency on the `PermissionIntegrationsService` to your plugin,
+and pass it the same options:
+
+```ts
+export const examplePlugin = createBackendPlugin({
+  pluginId: 'example',
+  register(env) {
+    env.registerInit({
+      deps: {
+        logger: coreServices.logger,
+        /* highlight-add-next-line */
+        permissionIntegrations: coreServices.permissionIntegrations,
+      },
+      /* highlight-remove-next-line */
+      async init({ logger }) {
+        /* highlight-add-next-line */
+      async init({ logger, permissionIntegrations }) {
+        logger.log('This is a silly example plugin with no functionality');
+
+        /* highlight-add-start */
+        permissionIntegrations.addResourceType({
+          resourceType: RESOURCE_TYPE_MY_RESOURCE,
+          permissions: [myResourcePermissions],
+          rules: [myResourceRule],
+        });
+        /* highlight-add-end */
+      },
+    });
+  },
+});
+```
+
+If you only passed the `permissions` option to
+`createPermissionIntegrationRouter`, you will want to use
+`permissionIntegrations.addPermissions` instead.
+
+If you passed multiple resources types to `createPermissionIntegrationRouter`
+via the `resources` option, you will want to call
+`permissionIntegrations.addResourceType` multiple times for each of those
+resource types.