style: Added WARN_LEVEL doc to README.md #3

Job	Run time
test-examples	30s
code-quality	15s
build (windows-latest, release)	3m 7s
build (ubuntu-latest, release)	19s
build (windows-latest, debug)	1m 33s
build (ubuntu-latest, debug)	25s
build (macos-latest, release)	9s
build (macos-latest, relwithdebinfo)	11s
build (ubuntu-latest, relwithdebinfo)	17s
build (windows-latest, relwithdebinfo)	2m 20s
build (macos-latest, debug)	12s
	9m 18s

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