Add note on HHVM support to readme
Add note on HHVM support to readme

marktopper/Stringy,danielkesselberg/Stringy,taohaoge/Stringy,bocharsky-bw/Stringy,Big-Shark/Stringy,takobell/Stringy,danielstjules/Stringy,TCB13/Stringy,wallacesilva/Stringy,lucasmichot/Stringy,ihsanudin/Stringy,sofyansitorus/Stringy,voku/Stringy,ycms/Stringy