Expand payjoin-service README

Author: spacebear21

payjoin-service/README.md

@@ -3,3 +3,52 @@
 Unified Payjoin Directory and OHTTP Relay service. Combines [payjoin-directory](../payjoin-directory/README.md) and [ohttp-relay](../ohttp-relay/README.md) into a single binary.
 
 Note that this binary is under active development and thus the CLI and configuration file may be unstable.
+
+## Configuration
+
+payjoin-service reads configuration from `config.toml` (or the path given with `--config`). Every setting can also be supplied via environment variables prefixed with `PJ_`, using double underscores for nesting (e.g., `PJ_TELEMETRY__ENDPOINT`).
+
+## Running the service
+
+### Cargo
+
+```sh
+cargo run
+```
+
+### Docker Compose
+
+A simple [docker-compose.yml](docker-compose.yml) is provided for convenience.
+
+```sh
+docker compose up
+```
+
+### Nix
+
+The rust-payjoin flake also provides `payjoin-service` as a package.
+
+```sh
+nix run .#payjoin-service -- --config payjoin-service/config.toml
+```
+
+## Telemetry
+
+payjoin-service supports **optional** OpenTelemetry-based telemetry (metrics, traces, and logs). Build with `--features telemetry` and add a `[telemetry]` section to your config:
+
+```toml
+[telemetry]
+endpoint = "https://otlp-gateway-prod-us-west-0.grafana.net/otlp"
+auth_token = "<base64 instanceID:token>"
+operator_domain = "your-domain.example.com"
+```
+
+Or set the equivalent environment variables:
+
+```sh
+export PJ_TELEMETRY__ENDPOINT="https://otlp-gateway-prod-us-west-0.grafana.net/otlp"
+export PJ_TELEMETRY__AUTH_TOKEN="<base64 instanceID:token>"
+export PJ_TELEMETRY__OPERATOR_DOMAIN="your-domain.example.com"
+```
+
+When no `[telemetry]` section is present and no `PJ_TELEMETRY__*` variables are set, the service uses local-only console tracing.