Change CONTRIBUTING.md to CONTRIBUTING.adoc

Description

Change CONTRIBUTING.md to CONTRIBUTING.adoc

Please ensure the following points are checked:

[ ] My code follows the coding style
[ ] I have reviewed file licenses
[ ] I have commented any added functions (in line with Doxygen)
[ ] I have commented any code that could be hard to understand
[ ] My changes do not add any new compiler warnings
[ ] My changes do not add any new validation layer errors or warnings
[ ] I have used existing framework/helper functions where possible
[ ] My changes do not add any regressions
[ ] I have tested every sample to ensure everything runs correctly
[ ] This PR describes the scope and expected impact of the changes I am making

Note: The Samples CI runs a number of checks including:
- [ ] I have updated the header Copyright to reflect the current year (CI build will fail if Copyright is out of date)
- [ ] My changes build on Windows, Linux, macOS and Android. Otherwise I have documented any exceptions

If your PR contains a new or modified sample, these further checks must be carried out in addition to the General Checklist:

[ ] I have tested the sample on at least one compliant Vulkan implementation
[ ] If the sample is vendor-specific, I have tagged it appropriately
[ ] I have stated on what implementation the sample has been tested so that others can test on different implementations and platforms
[ ] Any dependent assets have been merged and published in downstream modules
[ ] For new samples, I have added a paragraph with a summary to the appropriate chapter in the samples readme
[ ] For new samples, I have added a tutorial README.md file to guide users through what they need to know to implement code using this feature. For example, see conditional_rendering
[ ] For new samples, I have added a link to the Antora navigation so that the sample will be listed at the Vulkan documentation site