Add build-flutter template

templates/build-flutter

@@ -0,0 +1,34 @@
+# PLEASE REMOVE ALL COMMENTS BEFORE SUBMITTING TO F-DROID DATA!
+
+# Starting from the following template and grep for "flutter" in the metadata
+# directory to see other examples.
+
+Builds:
+  - versionName: '1.0'
+    versionCode: 1
+    commit: v1.0
+    output: build/app/outputs/flutter-apk/app-release.apk
+    # Choose one of the Tags of the Flutter source:
+    # https://github.com/flutter/flutter/tags.
+    # Sometimes the Flutter version is already specified by
+    # upstream in pubspec.yaml, environment.flutter.
+    srclibs:
+      - flutter@2.8.0
+    # Deletes unrelated directories,
+    # remove those not needed from the list.
+    rm:
+      - ios
+      - linux
+      - macos
+      - web
+      - windows
+    build:
+      - $$flutter$$/bin/flutter config --no-analytics
+      - $$flutter$$/bin/flutter build apk --release --verbose
+
+AutoUpdateMode: Version
+UpdateCheckMode: Tags
+UpdateCheckData: pubspec.yaml|version:\s.+\+(\d+)|.|version:\s(.+)\+
+CurrentVersion: '1.0'
+CurrentVersionCode: 1
+