Add Twilio sent, delivered and queued SMS events

[miike]

Replaces #688 #689 #690

[snowplowcla]

@miike has signed the Software Grant and Corporate Contributor License Agreement. Thanks so much

[alexanderdean]

Thanks @miike! I think we have a Twilio account... Do you have some configuration instructions so we can QA these schemas?

[miike]

Yes - will add this in tomorrow. What's the best way to share documentation relating to webhooks like this? Should there be a high level docs directory with Markdown READMEs?

[alexanderdean]

Great stuff - ultimately the webhook setup guide will just go into the wiki like so:

https://github.com/snowplow/snowplow/wiki/Setting-up-a-Webhook

It's probably easiest just to add the setup guide to this thread first and we can port over later...

[miike]

I've just had another look at the messaging service webhooks and it looks like I'll need to write a minimal adapter for this so that events don't all use the same schema.