[rocm-libraries] ROCm/rocm-libraries#4277 (commit 4348901)

Add a README.md file to ck/library/util
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I'm collecting information about our current testing (#3664). As part of
this work I a README to the directory to emphasize the GPU-first testing
strategy and our support for type-specific tolerances.

This readme contains internal code comments for CK developers and does
not need ROCm documentation review.