📚 docs: Add installation instructions

README.md

@@ -12,6 +12,65 @@ real primary instance unchanged, and forwarding the handshake and
 
 Linux and macOS only.
 
+## Install
+
+Download the latest build for your platform from the
+[Releases page](https://github.com/YuzuZensai/Discord-RPC-Relay/releases).
+
+### Linux (AppImage)
+
+1. Download `discord-rpc-relay-<version>.AppImage`.
+2. Make it executable and run it:
+
+   ```bash
+   chmod +x discord-rpc-relay-*.AppImage
+   ./discord-rpc-relay-*.AppImage
+   ```
+
+The app runs from the tray. To launch it automatically and have it appear in
+your application menu, move it somewhere permanent (e.g. `~/Applications/`)
+and create a desktop entry, or enable "Start on login" from the app's
+settings once it's running.
+
+### Linux (.deb)
+
+1. Download `discord-rpc-relay-<version>.deb`.
+2. Install it:
+
+   ```bash
+   sudo apt install ./discord-rpc-relay-*.deb
+   ```
+
+3. Launch it from your application menu ("Discord RPC Relay"), or from a
+   terminal:
+
+   ```bash
+   discord-rpc-relay
+   ```
+
+### macOS
+
+1. Download `discord-rpc-relay-<version>.dmg` and open it.
+2. Drag **Discord RPC Relay** into the **Applications** folder.
+3. Clear the Gatekeeper quarantine attribute before first launch (see below),
+   then open the app from Applications/Launchpad/Spotlight as usual.
+
+The app runs in the menu bar. Subsequent launches don't need the Gatekeeper
+step again.
+
+#### macOS Gatekeeper
+
+The macOS build isn't signed/notarized, so Gatekeeper will quarantine it and
+refuse to open it ("app is damaged and can't be opened" / "unidentified
+developer"). After installing, clear the quarantine attribute from the
+Terminal:
+
+```bash
+xattr -cr /Applications/Discord\ RPC\ Relay.app
+```
+
+Then open the app normally.
+
 ## Development
 
 ```bash