Add WIP Crashlytics guide

src/content/docs/guides/crash-reporting/crashlytics.mdx

@@ -0,0 +1,47 @@
+---
+title: Crashlytics Integration
+description: Integrate Shorebird into your Crashlytics crash reporting
+sidebar:
+  label: Crashlytics
+  order: 2
+---
+
+If you're using Crashlytics for crash reporting, it will work out-of-the-box
+with Shorebird releases and patches. However, if you have multiple patches, it
+can be unclear which patch caused the crash. This document shows how you can use
+Crashlytics to differentiate between patches.
+
+## Add the `shorebird_code_push` package to your project.
+
+[shorebird_code_push](https://pub.dev/packages/shorebird_code_push) is available
+on pub.dev and lets you programmatically determine your app's current patch
+number. To add it to your project, follow the instructions on the package's
+pub.dev page.
+
+## Configure Crashlytics
+
+If you haven't already, follow the Crashlytics [getting started with
+Flutter](https://firebase.google.com/docs/crashlytics/get-started?platform=flutter) guide.
+
+Update the Firebase init code to include the patch number as a custom key. This will look
+something like:
+
+```dart
+Future<void> main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+  await Firebase.initializeApp(
+    options: DefaultFirebaseOptions.currentPlatform,
+  );
+
+  final patchNumber = await ShorebirdCodePush().currentPatchNumber();
+
+  // Add the patch number as a tag. You can use whatever name you would like
+  // as the key. `$patchNumber` will be "null" if there is no patch. You may
+  // wish to handle this case differently.
+  FirebaseCrashlytics.instance.setCustomKey(
+    'shorebird_patch_number',
+    '$patchNumber',
+  );
+  runApp(const MyApp());
+}
+```