docs clarification

Updated README to clarify a bit more about the `twilioNumber` value. Without a leading '1' + area code + number as the value, the following error is shown in the console when attempting sign in:

```
{ status: 400,
  message: 'The \'From\' number +8885551212 is not a valid phone number or shortcode.',
  code: 21212,
  moreInfo: 'https://www.twilio.com/docs/errors/21212' }
```