Clarify when to use purchase/authorize/capture and confirm in README

[jamieburchell]

At the moment, the README reads like the purchase method is only for the Charges API. For the Payment Intents API it shows an example of using authorize and then just confirm, but no capture of the authorization. This is confusing to me, and I needed to read through the comments of the classes to understand when to use each and how to authorize and capture separately. The comments showing this seem to differ from the README example. It would be really helpful to show how to use the library for each use case. For example, how to authorise then capture later, when to use the confirm param: true/false (why might this be used) vs using the confirm method etc. The documentation also lacks detail on how to set a custom reference number to pick up later to retrieve payment details.