Improve docs on probot simulate command (#405)

docs/simulating-webhooks.md

@@ -8,10 +8,15 @@ As you are developing your app, you will likely want to test it by repeatedly tr
 
 To save a copy of the payload, go to the  [settings](https://github.com/settings/apps) page for your App, and go to the **Advanced** tab. Click on one of the **Recent Deliveries** to expand it and see the details of the webhook event. Copy the JSON from the the **Payload** and save it to a new file. (`test/fixtures/issues.labeled.json` in this example).
 
+**Note**: This will only simulate the JSON payload, not the headers that are also sent by GitHub webhooks.
+
 ![](https://user-images.githubusercontent.com/173/28491924-e03e91f2-6ebe-11e7-9570-6d48da68c6ca.png)
 
 Next, simulate this event being delivered by running:
 
-```
-$ node_modules/.bin/probot simulate issues test/fixtures/issues.labeled.json ./index.js
-```
+    $ node_modules/.bin/probot simulate <event-name> <path-to-fixture> <path-to-app>
+
+Note that `event-name` here is just the name of the event (like pull_request or issues) and not the action (like labeled). For example, to simulate the pull_request.labeled event, run:
+
+    $ node_modules/.bin/probot simulate pull_request test/fixtures/pull_request.labeled.json ./index.js
+