Add security policy #719

Job	Run time
cs-stan / Coding Standard & Static Analysis	39s
testsuite / testsuite-linux (8.2, pgsql, highest, false)	42s
testsuite / testsuite-linux (8.5, sqlite, highest, false)	37s
testsuite / testsuite-linux (8.2, sqlite, lowest, false)	35s
testsuite / testsuite-linux (8.2, mysql, highest, false)	48s
testsuite / testsuite-linux (8.4, mysql, highest, false)	43s
testsuite / testsuite-linux (8.3, sqlite, highest, false)	24s
	4m 28s

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