docs(README): update Smee usage documentation (#901)

GH-889: Update Smee Documentation

The current code in Smee documentation does not work as is, we need to stringify the body in order to properly do the signature verification process.

README.md

@@ -75,7 +75,7 @@ source.onmessage = (event) => {
       id: webhookEvent["x-request-id"],
       name: webhookEvent["x-github-event"],
       signature: webhookEvent["x-hub-signature"],
-      payload: webhookEvent.body,
+      payload: JSON.stringify(webhookEvent.body),
     })
     .catch(console.error);
 };