Add callback handling guide to README

[cmpaul]

We don't have any guidance on this, let's add a section to the README.

[desmondw]

Closing this because we have documentation regarding the handling of callbacks in our API docs and link to them in the README. https://www.hellosign.com/api/reference#Callbacks

[BHSPitMonkey]

I'm willing to leave this open, since there's more we can do here. Currently the only PHP example on the walkthrough is computing the hash, which is just one piece of the puzzle.

[garnold]

@BHSPitMonkey Also note that the PHP example you referenced could be simplified when using the SDK:

$event = new HelloSign\Event(json_decode($_POST['json']))
if (!is_null($event) && $event->isValid($apiKey)) {
    // do something useful
}

[josiahmann]

I'm just finishing my integration and I'll agree that documentation for this is lacking compared to the other existing documentation. Thanks for the comments though @garnold , that helped me clarify my setup.