CI for MySQL Tuner for MySQL 5.7 and 8.0

Update README.md, remove double 'by default' in some sentences. #200

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Run time

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Job	Run time
test_help (5.7)	41s
test_help (8)	40s
test_with_empty_db (5.7)	1m 12s
test_with_empty_db (8)	53s
	3m 26s