feat: Add initial README

Author: SunPodder

README.md

@@ -0,0 +1,47 @@
+# Relay Android App
+
+Relay is an Android app for cross-device notification and message relay, supporting a custom protocol for real-time communication with desktop clients.
+
+## Features
+
+- Notification mirroring from Android to desktop
+- Actionable notifications (reply, action buttons, dismiss)
+- Bidirectional protocol with length-prefixed JSON messages
+
+## Getting Started
+
+**Install the APK**: Download the latest APK from the [Releases page](https://github.com/SunPodder/Relay/releases) and install it on your Android device.
+
+Or **Build from source**: Clone this repository and build the app using Android Studio or Gradle:
+```bash
+./gradlew assembleDebugOptimized
+```
+Then install the generated APK on your device.
+
+**First-time setup:**
+
+- Open the Relay app after installation.
+- Grant the required notification permissions when prompted.
+- Go to your device's App Settings for Relay → Battery optimizations → Allow unrestricted usage (to ensure background operation).
+- Make sure the app server is running.
+- You can now close the app; the Relay service will keep running in the background and forward notifications to connected clients as long as permissions are granted.
+
+## Cross-Platform Desktop Client
+
+A cross-platform desktop client implementation is available at:
+- https://github.com/SunPodder/relay-pc
+
+This client supports Windows, Linux, and macOS, and is designed to work seamlessly with this Android app.
+
+## Protocol
+
+- All communication uses a 4-byte big-endian length-prefixed JSON envelope.
+- See `protocols/` for protocol details.
+
+## Contributing
+
+Pull requests and issues are welcome!
+
+## License
+
+MIT License