Document the use of sdpfilter

[silviapfeiffer]

The options in quickconnect can contain an sdpfilter like so:
{
room: channelId,
iceServers: config.connection.iceServers,
sdpfilter: function(sdp) {
console.log('filtering sdp');
return limitBandwidthSDP(sdp);
}
}

Should be documented (maybe even add to the book)

[silviapfeiffer]

Alternatively, we could add actual settings for specific things like bandwidthLimit, but that's non-standard.

[magestican]

this is still missing from the documentation

[silviapfeiffer]

@magestican feel free to add a pull request