RATIS-2305. Add a security doc. #57

Job	Run time
CI / release / release	5m 19s
CI / build / build	1m 25s
CI / basic (rat) / rat	23s
CI / basic (author) / author	8s
CI / basic (checkstyle) / checkstyle	28s
CI / unit (misc) / unit (misc)	15m 0s
CI / unit (server) / unit (server)	19m 4s
CI / basic (findbugs) / findbugs	2m 1s
CI / unit (grpc) / unit (grpc)	22m 38s
CI / unit (flaky) / unit (flaky)	5m 24s
CI / repro / repro	1m 13s
CI / compile (17) / compile (17)	1m 1s
CI / compile (11) / compile (11)	1m 0s
CI / compile (21) / compile (21)	1m 3s
CI / coverage	0s
CI / build / build	1m 25s
CI / release / release	5m 19s
CI / basic (rat) / rat	23s
CI / basic (checkstyle) / checkstyle	28s
CI / basic (author) / author	8s
CI / unit (grpc) / unit (grpc)	21m 10s
CI / unit (flaky) / unit (flaky)	5m 24s
CI / unit (misc) / unit (misc)	15m 0s
CI / basic (findbugs) / findbugs	2m 1s
CI / unit (server) / unit (server)	19m 4s
CI / repro / repro	1m 13s
CI / compile (21) / compile (21)	1m 3s
CI / compile (11) / compile (11)	1m 0s
CI / compile (17) / compile (17)	1m 1s
CI / coverage	27s
	2h 31m 13s

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