Fill out hello-world example

docs/hello-world.md

@@ -13,3 +13,32 @@ module.exports = robot => {
 ```
 
 The `robot` parameter is an instance of [`Robot`](https://probot.github.io/probot/latest/Robot.html) and gives you access to all of the bot goodness.
+
+`robot.on` will listen for any [webhook events triggered by GitHub](./webhooks.md), which will notify you when anything interesting happens on GitHub that your plugin wants to know about.
+
+```js
+module.exports = robot => {
+  robot.on('issues.opened', async context => {
+    // A new issue was opened, what should we do with it?
+    robot.log(context);
+  });
+};
+```
+
+The `context` passed to the event handler includes everything about the event that was triggered, as well as some helpful properties for doing something useful in response to the event. `context.github` is an authenticated GitHub client that can be used to [make API calls](./github-api.md), and allows you to do almost anything programmatically that you can do through a web browser on GitHub.
+
+Here is an example of an autoresponder plugin that comments on opened issues:
+
+```js
+module.exports = robot => {
+  robot.on('issues.opened', async context => {
+    // `context` extracts information from the event, which can be passed to
+    // GitHub API calls. This will return:
+    //   {owner: 'yourname', repo: 'yourrepo', number: 123, body: 'Hello World!}
+    const params = context.issue({body: 'Hello World!'})
+
+    // Post a comment on the issue
+    return context.github.issues.createComment(params);
+  });
+}
+```

docs/webhooks.md

@@ -17,7 +17,7 @@ module.exports = robot => {
 };
 ```
 
-The robot can listen to any of the [GitHub webhook events](https://developer.github.com/webhooks/#events). The `context` object includes all of the information about the event that was triggered, and `context.payload` has the payload delivered by GitHub.
+The robot can listen to any of the [GitHub webhook events](https://developer.github.com/webhooks/#events). The `context` object includes everything about the event that was triggered, and `context.payload` has the payload delivered by GitHub.
 
 Most events also include an "action". For example, the [`issues`](https://developer.github.com/v3/activity/events/types/#issuesevent) event has actions of `assigned`, `unassigned`, `labeled`, `unlabeled`, `opened`, `edited`, `milestoned`, `demilestoned`, `closed`, and `reopened`. Often, your bot will only care about one type of action, so you can append it to the event name with a `.`: