Update README.md with usage example

[hcmlopes]

Adds example to discountCode.create to show that it does not need the full "discount_code" object as the param as shown in the shopify API docs

[coveralls]

Coverage remained the same at 100.0% when pulling 083706a on hcmlopes:patch-1 into e64c6d3 on MONEI:master.

[coveralls]

Coverage remained the same at 100.0% when pulling c26947c on hcmlopes:patch-1 into e64c6d3 on MONEI:master.

[lpinca]

This is valid for all resources not only DiscoutCode. I would prefer a paragraph with an example under the Resources section to make it extra clear.

[lpinca]

Thank you.