Create `CONTRIBUTING.md` #577

Job	Run time
msrv	1m 36s
test	1m 30s
check format	12s
check	18s
check (--no-default-features)	7s
check (--no-default-features --features use_alloc)	24s
check (--all-targets --all-features)	21s
All checks succeeded	1s
	4m 29s

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