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add dev banner to docs #769

Job	Run time
cs-stan / Coding Standard & Static Analysis	39s
testsuite / testsuite-linux (8.4, sqlite, lowest, false)	36s
testsuite / testsuite-linux (8.5, mysql, highest, false)	52s
testsuite / testsuite-linux (8.5, sqlite, highest, false)	28s
testsuite / testsuite-linux (8.4, pgsql, highest, false)	41s
testsuite / testsuite-linux (8.4, mysql, highest, false)	53s
	4m 9s