shim: nicer submit key message

Suggests user cli args to specify the submit private key.

Closes #109

Author: pepyakin

sugondat-shim/src/cmd/serve.rs

@@ -10,7 +10,10 @@ pub async fn run(params: Params) -> anyhow::Result<()> {
     let listen_on = (params.dock.address.as_str(), params.dock.port);
     let submit_key = crate::cmd::load_key(params.key_management)?;
     if submit_key.is_none() {
-        tracing::info!("no submit key provided, will not be able to submit blobs");
+        tracing::info!(
+            "no submit key provided, will not be able to submit blobs. \
+Pass --submit-dev-alice or --submit-private-key=<..> to fix."
+        );
     }
     let server = Server::builder().build(listen_on).await?;
     let client = connect_client(&params.rpc.node_url).await?;