Document how to run simple_webrtc better

[jdm]

Some changes I had to make to get it to work:

• run simple_server.py with --disable-ssl (kept getting cert errors in the browser otherwise)
• modify webrtc.js to use ws:// instead of wss://
• https://github.com/servo/media/pull/346
• cd examples; cargo run --bin simple_webrtc --features=webrtc

[Manishearth]

Note: I've been using https://github.com/centricular/gstwebrtc-demos/compare/master...Manishearth:servo for webrtc stuff