Update android.md

Author: Salakar

docs/app/android.md

@@ -3,9 +3,43 @@ title: Android Initialization
 description: Initialization Firebase with Android. 
 ---
 
-# Android Initialization
+# Android Setup
+
+## Manual Android Integration
 
 > The following steps are only required if your environment does not have access to React Native
-auto-linking. 
+auto-linking.
+
+#### Add App to Gradle Settings
+
+**`android/settings.gradle`**:
+```groovy
+include ':@react-native-firebase_app'
+project(':@react-native-firebase_app').projectDir = new File(rootProject.projectDir, './../node_modules/@react-native-firebase/app/android')
+```
+
+#### Add App to App Gradle Dependencies
+
+**`android/app/build.gradle`**:
+```groovy{4}
+// ..
+dependencies {
+  // ..
+  implementation project(path: ":@react-native-firebase_app")
+}
+```
+
+#### Add App to Main Android Application:
+
+**`android/app/src/main/java/**/MainApplication.java`**:
+```java{2,8}
+// ..
+import io.invertase.firebase.app.ReactNativeFirebaseAppPackage;
 
-## TODO
+  // ..
+  protected List<ReactPackage> getPackages() {
+    return Arrays.asList(
+      new MainReactPackage(),
+      new ReactNativeFirebaseAppPackage(),
+      // ..
+```