feat(webrtc): add WebRTC (prev. browser-to-browser) spec (#497)

Introduces the webrtc protocol - a libp2p transport protocol enabling two
private nodes (e.g. two browsers) to establish a direct connection.

Author: mxinden

README.md

@@ -99,7 +99,7 @@ see [#465](https://github.com/libp2p/specs/issues/465).
 - [secio][spec_secio] - SECIO, a transport security protocol for libp2p
 - [tls][spec_tls] - The libp2p TLS Handshake (TLS 1.3+)
 - [quic][spec_quic] - The libp2p QUIC Handshake
-- [webrtc][spec_webrtc] - The libp2p WebRTC transport
+- [webrtc][spec_webrtc] - The libp2p WebRTC transports
 - [WebTransport][spec_webtransport] - Using WebTransport in libp2p