Docs: Add CONTRIBUTING guide with checks and tips

CONTRIBUTING.md

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+Contributing
+
+Thanks for your interest in contributing to Ver-ID UI for Android!
+
+Before you open a pull request:
+
+- Ensure `./gradlew :veridui:lint :veridui:test` passes
+- Build `:veridui` and `:sample` locally (or let CI validate)
+- Follow the PR template and keep changes focused and scoped
+
+Development tips
+
+- Use Android Studio Giraffe or newer with JDK 17
+- Run the sample app on a physical device for camera features
+
+Project scripts
+
+- Translation completeness check: `python3 test_translation.py veridui/src/main/assets/<locale>.xml`
+