add preprocessor wiki link to readme
add preprocessor wiki link to readme

dockbiz/msgp,appneta/msgp,jfbus/msgp,kgrz/msgp,eliothedeman/msgp,ebfe/msgp,darvik80/msgp,tinylib/msgp,appneta/msgp,tinylib/msgp,mengzhuo/msgp,skatsuta/msgp,segmentio/msgp,DrGo/msgp