nonce missing in the Verifiable Credentials example

[Sakurann]

nonce should be mandatory to be included in the returned Verifiable Presentation and binds it to a transaction. (nonce might not be explicitly mentioned in the W3C vc-data-model, but it is because how to transport VCs is out of scope of that document).
(I included nonce in the example in the Issue #5)

[samuelgoto]

Good point!

Done.