Add Signed SMS Environment Variables

[dragonmantank]

We need to add FROM_NUMBER and VONAGE_API_SIGNATURE_SECRET to the .env-example file so that they can be set when users run sms/send-signed-sms.php

[moisesortega]

Hi, I'm fairly new to open source, and I was wondering if I could work on this for Hacktoberfest?

[dragonmantank]

Of course! Any help is welcomed!