README.md: shift description of kernel config files

[mhorne]

Summary

Document it in sys/README.md instead. Describe the purpose of LINT
config files.

[mhorne]

sys/README.md

10

I am not determined that this information must be kept here, but I found the "sys/arch/conf" entry a little awkward.

[imp]

sys/README.md

10

It's fine, though I want to quibble over the wording here, I've got nothing better to suggest.
I like your awkward construct better, but the proposed change is also fine.
GENERIC isn't 100% correct, but it's a good generalization and I don't think we need 100% correctness here (though now that I thin about it, only removed architectures are the exception)