Add suuuper-simple dev instructions

[philschatz]

Since Pull Requests are auto-deployed the friction to contribute is greatly reduced. Now, you can test the bot by just using the GitHub.com web interface... no need for getting dev keys or cloning the repository on your local machine.

Screencap of adding a new Feature

Here is the code for the New Feature we write in the Screencap:

// Add a "Hello!" message when a new Issue is created
robot.on(['issues.opened'], async(context) => {
  await context.github.issues.createComment(context.issue({body: 'Hello!'}))
})
// We will test it by creating an Issue in https://github.com/philschatz/test

Screenshot of Review apps running

And here is what the Heroku dashboard looks like with multiple Pull Requests open. It shows the Review Apps on the left (the deployed Pull Requests) and the Production app on the right.