Add notice about Autobahn|JS 0.9 to readme

[bjoern-tantau]

After some frustrating hours I found out that Autobahn|JS 0.9 is not supported by Ratchet. You have to use Version 0.8. Maybe you should add a note to the documentation.

[sidneywidmer]

Thanks for your input, I'll add that to docs as soon as possible. As long as Ratchet doesn't support WAMP 2 (ratchetphp/Ratchet#168), we're limited to the Basic Profile.