update README.md to mark mDNS complete
- sync up to [pion/dtls/v2.0.0](https://github.com/pion/dtls/tree/789798433596e4dd92451b66984dddb2f8a9f165)
- [x] STUN: [webrtc-rs/stun](https://github.com/webrtc-rs/stun) [![stun][stun-badge]][stun-url]
- sync up to [pion/stun/v0.3.5](https://github.com/pion/stun/tree/7b20b792b7e18b3846032aaa80e8c0e2d412d0f8)
-- [ ] mDNS: [webrtc-rs/mdns](https://github.com/webrtc-rs/mdns) [![mdns][mdns-badge]][mdns-url]
- - work in progress
- - catch up [pion/mdns/v0.0.4](https://github.com/pion/mdns/tree/2e1665e5f21a89afc152bb4b3791b30eda9b28cf)
+- [x] mDNS: [webrtc-rs/mdns](https://github.com/webrtc-rs/mdns) [![mdns][mdns-badge]][mdns-url]
+ - sync up to [pion/mdns/v0.0.4](https://github.com/pion/mdns/tree/2e1665e5f21a89afc152bb4b3791b30eda9b28cf)
- [ ] TURN: [webrtc-rs/turn](https://github.com/webrtc-rs/turn) [![turn][turn-badge]][turn-url]
- work in progress
- catch up [pion/turn/v2.0.2](https://github.com/pion/turn/tree/502d01577bf86a442ab9b9fa23f78987e7f1e1cd)
