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add `SECURITY.md`, pointing to GitHub PVR #87

Job	Run time
Cygwin	2m 11s
Linux (default, valgrind, test-memory)	3m 5s
Windows	1m 4s
macOS	21s
Linux (clang sanitizers, clang, test, clang, -fsanitize=address,undefined,signed-integer-overflow...	28s
	7m 9s

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