Add git submodule instructions to readme #219

Job	Run time
Build	54s
Verify SSM Agent I/O independence bugs	2m 59s
Verify SSM Agent protocol model	7m 7s
Verify DH core	1m 18s
Verify SSM Agent core assumptions bugs	5m 43s
Verify SSM Agent core assumptions	7m 34s
Verify DH I/O independence bugs	1m 8s
Verify DH protocol model	1m 8s
Verify DH core assumptions	1m 8s
Verify SSM Agent I/O independence	1m 58s
Verify SSM Agent core	4m 7s
Verify DH I/O independence	1m 24s
Verify DH core assumptions bugs	1m 6s
Publish	1m 7s
Cleanup	4s
	38m 45s

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