document how to use (and validate) webhooks

We should update the README with an example of how to set a webhook.

This page has some prior art: https://replicate.com/docs/webhooks#setting-webhooks

```js
await replicate.predictions.create({
  version: "d55b9f2d...",
  input: { prompt: "call me later maybe" },
  webhook: "https://example.com/replicate-webhook",
  webhook_events_filter: ["completed"], // optional
});
```