docs: add note about cncf to readme #5236

Job	Run time
sample_operators_tests (sample-operators/tomcat-operator)	3m 2s
sample_operators_tests (sample-operators/leader-election)	3m 9s
sample_operators_tests (sample-operators/mysql-schema)	2m 31s
sample_operators_tests (sample-operators/webpage)	2m 51s
	11m 33s

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