No license and attribution for XML files from Opcodes DB

[Maratyszcza]

Nice that you found a use for my Opcodes database, but you should follow the terms of the license.

[mattbierner]

I've added an acknowledgement that x86_64.xml came from the Opcodes project and linked back to it. Thanks for putting it together by the way, it was by far the easiest x86 documentation to consume for a simple use case like this project.

[Maratyszcza]

Thank you!