Add Vendr.Checkout docs about what property names it uses

[mattbrailsford]

When using Vendr.Checkout it uses a number of hard coded property aliases for things like billing and shipping address. In some payment providers, these property aliases need to be referenced in the providers settings but we don't currently publicize what these property aliases are.

We should add some docs to list the property aliases uses so people can reference them should they need them.