Create `CONTRIBUTING.md` #462

Job	Run time
check format	11s
msrv	1m 15s
check	14s
test	1m 17s
check (--no-default-features)	16s
check (--no-default-features --features use_alloc)	20s
check (--all-targets --all-features)	41s
bors build finished	1s
	4m 15s

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