Add CONTRIBUTING.md pointing to the review guidelines #536

Job	Run time
Build the fuzzers	48s
Run the fuzzers (fuzz_cmp, true)	1m 36s
Run the fuzzers (fuzz_cmp_args, true)	1m 43s
Run the fuzzers (fuzz_ed, true)	1m 31s
Run the fuzzers (fuzz_patch, true)	1m 33s
Run the fuzzers (fuzz_normal, true)	1m 46s
Run the fuzzers (fuzz_side, true)	1m 45s
	10m 42s

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