README: Document which streams are provided automatically via webrtc

[patrickelectric]

At the moment the readme is not clear when a webrtc stream is provided automatically by a source.
E.g: A RTSP redirect will also be provided as a webrtc.

[joaoantoniocardoso]

maybe in the key-features section, change it as in:

--- WebRTC-ready, for modern control stations
+++ WebRTC-ready, for modern control stations, available from any H264 source.

what do you think?

[patrickelectric]

Perfect!