Alphanumeric Sender IDs missing documentation

[ace-of-aces]

trafficstars

When I tried adding an alphanumeric sender ID to a Twilio notification, I had to dig through the source code of this package to find out that the Notifiable must have a canReceiveAlphaNumericSender function that returns true in order for the sender ID to be respected. I could not find this anywhere in the documentation.

Also, the PHPDoc description of the from method in the TwilioMessage only states "Set the phone number the message should be sent from.", although that this could also be an alphanumeric sender ID.

I found this to be quite inconvenient to work with, so I would propose for this to be properly documented. I'd be happy to create PRs for these!