docs/README: add contribution guidelines #72

Job	Run time
deadnix	53s
nix flake show	31s
nixfmt	1m 29s
nix flake check	1m 31s
nix build .#bgp-extended-nexthop	1m 9s
nix build .#bgp-frr-unnumbered	1m 37s
nix build .#bgp-bird-tcpao	1m 41s
nix build .#bgp-simple	1m 42s
nix build .#bgp-vrf	1m 53s
nix build .#bgp-md5	1m 17s
nix build .#bgp-ttl-security	1m 34s
nix build .#dns-knot-dnssec	1m 35s
nix build .#dns-knot	1m 33s
nix build .#bgp-prefsource	1m 35s
nix build .#dhcpv4	1m 46s
nix build .#dns-knot-xfr-tsig-explicit-notify	1m 50s
nix build .#dns-knot-xfr-dnssec	1m 39s
nix build .#dns-knot-xfr	1m 36s
nix build .#dns-knot-xfr-tsig	1m 38s
nix build .#nat64-dns64	1m 46s
nix build .#ping6-local-link	1m 43s
nix build .#radvd	1m 29s
	33m 27s

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