Setup key signing

RELEASE.md

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+# Release Process
+
+## Android Play Store
+
+[Reference](https://flutter.dev/docs/deployment/android#signing-the-app)
+
+### Key File
+
+Add a file `key.properties` under the android/ directory
+
+### Increment Build Number
+
+Make sure that the build number is incremented every time (or it will be rejected by Play Store).
+
+### Build Appbundle
+
+`flutter build appbundle`