Add iOS and Android app bundle builds

Solving issue https://github.com/subosito/flutter-action/issues/7 .

README.md

@@ -18,6 +18,42 @@ steps:
 - run: flutter build apk
 ```
 
+Build for iOS too (macOS only):
+
+```yaml
+jobs:
+  build:
+    runs-on: macos-latest
+    steps:
+    - uses: actions/checkout@v1
+    - uses: actions/setup-java@v1
+      with:
+        java-version: '12.x'
+    - uses: subosito/flutter-action@v1
+      with:
+        flutter-version: '1.9.1+hotfix.6'
+    - run: flutter pub get
+    - run: flutter test
+    - run: flutter build apk
+    - run: flutter build ios --release --no-codesign
+```
+
+Use app bundle, instead of APK:
+
+```yaml
+steps:
+- uses: actions/checkout@v1
+- uses: actions/setup-java@v1
+  with:
+    java-version: '12.x'
+- uses: subosito/flutter-action@v1
+  with:
+    flutter-version: '1.9.1+hotfix.6'
+- run: flutter pub get
+- run: flutter test
+- run: flutter build appbundle
+```
+
 Use latest release for particular channel:
 
 ```yaml