added new PR info to the README

It would be more work, but it would be pretty cool to make this a bit more ProMotion-like:

```ruby
{
  key: "password",
  label: "New Password",
  type: :password,
  textLabel: {
    color: UIColor.blueColor
  },
  value: ""
}
```

Not saying you should change this, necessarily. It would be cool in a future PR.