Warning of incompatibility with 0.10

[bfirsh]

I spent quite a lot of time trying to getting a websocket proxy working on 0.10 before digging through the issues and realising 0.10 isn't supported. Maybe it would be worth adding a note to the readme that node-http-proxy is only compatible with 0.8?

[robertjpayne]

+1 This is a pretty major feature break with no notes at all until digging around in the issues list.