Docs for subscriptions

[RogerSelwyn]

To add documentation in support of subscriptions

[RogerSelwyn]

I've significantly changed the content and then referenced the example app in the examples directory. I've also removed some of the validation of numbers from the example code, since that removes focus from the core code that makes it work.

Reality is this could actually be cut more to remove all but the webhook url, since that is the only one that is actually needed to make the subscriptions work. Everything else can be initiated in code if needed. I'm open to thoughts on this.

[RogerSelwyn]

Marking ready for review and comment. I'll take any suggestions.

[alejcas]

Why adding flask a dev requirement?

[RogerSelwyn]

I think I added it to support testing subscriptions. I believe it can be removed.

[alejcas]

Thanks @RogerSelwyn, merged!