Add contributing documentation

docs/contributing/sub-projects.md

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+The sing-box uses the following projects which also need to be maintained:
+
+#### sing
+
+Link: [GitHub repository](https://github.com/SagerNet/sing)
+
+As a base tool library, there are no dependencies other than `golang.org/x/sys`.
+
+#### sing-dns
+
+Link: [GitHub repository](https://github.com/SagerNet/sing-dns)
+
+Handles DNS lookups and caching.
+
+#### sing-tun
+
+Link: [GitHub repository](https://github.com/SagerNet/sing-tun)
+
+Handle Tun traffic forwarding, configure routing, monitor network and routing.
+
+This library needs to periodically update its dependency gVisor (according to tags), including checking for changes to
+the used parts of the code and updating its usage. If you are involved in maintenance, you also need to check that if it
+works or contains memory leaks.
+
+#### sing-shadowsocks
+
+Link: [GitHub repository](https://github.com/SagerNet/sing-shadowsocks)
+
+Provides Shadowsocks client and server
+
+#### sing-vmess
+
+Link: [GitHub repository](https://github.com/SagerNet/sing-vmess)
+
+Provides VMess client and server
+
+#### netlink
+
+Link: [GitHub repository](https://github.com/SagerNet/netlink)
+
+Fork of `vishvananda/netlink`, with some rule fixes.
+
+The library needs to be updated with the upstream.
+
+#### quic-go
+
+Link: [GitHub repository](https://github.com/SagerNet/quic-go)
+
+Fork of `lucas-clemente/quic-go`  and `HyNetwork/quic-go`, contains quic flow control and other fixes used by Hysteria.
+
+Since the author of Hysteria does not follow the upstream updates in time, and the provided fork needs to use replace,
+we need to do this.
+
+The library needs to be updated with the upstream.
+
+#### certmagic
+
+Link: [GitHub repository](https://github.com/SagerNet/certmagic)
+
+Fork of `caddyserver/certmagic`
+
+Since upstream uses `miekg/dns` and we use `x/net/dnsmessage`, we need to replace its DNS part with our own
+implementation.
+
+The library needs to be updated with the upstream.
+
+#### smux
+
+Link: [GitHub repository](https://github.com/SagerNet/smux)
+
+Fork of `xtaci/smux`
+
+Modify the code to support the writev it uses internally and unify the buffer pool, which prevents it from allocating
+64k buffers for per connection and improves performance.
+
+Upstream doesn't seem to be updated anymore, maybe a replacement is needed.
+
+Note: while yamux is still actively maintained and better known, it seems to be less performant.