Improve README with configuration, build, and usage info

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -1,8 +1,27 @@
-# Alert-to-notify
+# alerttonotify
 
-Receives HTTP requests from alertmanager. Sends to DBUS notify.
+Receives Alertmanager webhook notifications and forwards them via NATS to a dbus notification service.
 
-# Usage
+## Configuration
 
-TODO
+All configuration is via environment variables:
 
+| Variable | Required | Description |
+|---|---|---|
+| `NATS_URL` | Yes | NATS server URL |
+| `NATS_NKEY` or `NATS_NKEY_FILE` | Yes | NKey seed for NATS authentication |
+| `ALERTTONOTIFY_ADDR` | No | HTTP listen address (default `:5001`) |
+
+## Building
+
+```bash
+# With Go
+go build -v ./...
+
+# With Nix
+nix build
+```
+
+## Usage
+
+Point an Alertmanager webhook receiver at `http://<host>:5001/alert`.