Update readme

README.md

@@ -1,15 +1,18 @@
-# Validation Relay
+# registration-relay
+A relay that helps the [iMessage bridge] fetch data from registration providers ([Mac], [iOS]).
 
-## Build
+[iMessage bridge]: https://github.com/beeper/imessage
+[Mac]: https://github.com/beeper/mac-registration-provider
+[iOS]: https://github.com/beeper/phone-registration-provider
 
-Local binary:
+## Usage
+Build the binary (clone + `go build`) or use a Docker image from GHCR (`ghcr.io/beeper/registration-relay`),
+then just run it with some environment variables for configuration.
 
-```sh
-go build ./cmd/registration_relay
-```
+* `REGISTRATION_RELAY_SECRET` (required) - 32 byte secret key as base64, used to authenticate providers when reconnecting.
+    * A secret can be generated with `openssl rand -base64 32`
+* `REGISTRATION_RELAY_LISTEN` (defaults to :8000) - IP and port to listen on.
+* `REGISTRATION_RELAY_METRICS_LISTEN` (defaults to :5000) - IP and port to listen on for Prometheus metrics.
 
-Docker image:
-
-```sh
-docker build .
-```
+A reverse proxy configured to allow websockets should be pointed at the RELAY_LISTEN port to enable TLS.
+The bridge and registration providers can then be pointed at the public address of the reverse proxy.