Add docs on using FlutterFlow with custom connector (#183)

* Add docs on using FlutterFlow with custom connector

* Add a note to the introduction

---------

Co-authored-by: benitav <benita@journeyapps.com>

Author: simolus3

integration-guides/flutterflow-+-powersync.mdx

@@ -24,6 +24,10 @@ Used in conjunction with **FlutterFlow**, PowerSync enables developers to build
   Note that using libraries in FlutterFlow requires being on a [paid plan with FlutterFlow](https://www.flutterflow.io/pricing). If this is not an option for you, you can use our [legacy guide](/integration-guides/flutterflow-+-powersync/powersync-+-flutterflow-legacy) with custom code to integrate PowerSync in your FlutterFlow project.
 </Tip>
 
+<Note>
+ This guide uses **Supabase** as the backend database provider for its seamless integration with PowerSync. However, you can integrate a different backend using custom actions. For more information, refer to the [Custom backend connectors](#custom-backend-connectors) section.
+</Note>
+
 ## Guide Overview
 
 <Note>
@@ -595,7 +599,7 @@ bucket_definitions:
   **Checkpoint:** Your app should continue running seamlessly as before.
 </Check>
 
-## Arrays, JSON and other types
+## Arrays, JSON and Other Types
 
 For column values, PowerSync supports three basic types: Integers, doubles, and strings. These types have been chosen because
 they're natively supported by SQLite while also being easy to transport as JSON.
@@ -671,6 +675,48 @@ This allows you to customize how individual values are represented for Postgres.
 column of the `lists` table is decoded as JSON so that it's uploaded as a proper array while being stored
 as a list locally.
 
+## Custom Backend Connectors
+
+To enable an easy setup, the PowerSync FlutterFlow library integrates with Supabase by default. This means
+that as long as you use Supabase for authentication in your app, PowerSync will automatically connect as
+soon as users log in, and can automatically upload local writes to a Supabase database.
+
+For apps that don't use Supabase, you can disable this default behavior and instead rely on your own
+backend connectors.
+For this, create your own custom action (e.g. `applyPowerSyncOptions`). It's important that this action runs
+before anything else in your app uses PowerSync, so add this action to your `main.dart` as a final action.
+
+```dart
+import 'package:power_sync_b0w5r9/custom_code/actions/initialize_power_sync.dart';
+import 'package:powersync/powersync.dart' as ps;
+
+Future applyPowerSyncOptions() async {
+  // Disable the default Supabase integration
+  powerSyncOptions.useSupabaseConnector = false;
+  final db = await getOrInitializeDatabase();
+
+  // TODO: Write your own connector and call connect/disconnect when a user logs
+  // in.
+  db.connect(connector: _MyCustomConnector());
+}
+
+final class _MyCustomConnector extends ps.PowerSyncBackendConnector {
+  @override
+  Future<ps.PowerSyncCredentials?> fetchCredentials() {
+    // TODO: implement fetchCredentials
+    throw UnimplementedError();
+  }
+
+  @override
+  Future<void> uploadData(ps.PowerSyncDatabase database) {
+    // TODO: implement uploadData
+    throw UnimplementedError();
+  }
+}
+```
+
+For more information on writing backend connectors, see [integrating with your backend](/client-sdk-references/flutter#3-integrate-with-your-backend).
+
 ## Known Issues, Limitations and Gotchas
 
 Below is a list of known issues and limitations.