README.md is misleading on spec copyright

[oddhack]

The license section says

License: mixed — approximately: proprietary Khronos license for the specification Creative Commons Attribution 4.0 for document sources Apache 2 for code

which could mislead people. I suggest replacing this with:

• Proprietary Khronos license for the Vulkan Specification published by Khronos on its website
• CC-BY 4.0 for asciidoctor sources for the Specification and related documents such as the reference pages, style guide, and XML schema documentation.
• Apache 2 for code, including the XML registry defining API interfaces

[KarenGhavam-lunarG]

Thanks Jon for your clarification. I will push this change for the Vulkan Specification in the Map of Vulkan Ecosystem projects.