Doc: Small optimization on README.md

[wsd1]

In README.md, "$GOPATH/bin/go-oryx -c conf/oryx.json" can be changed to "$GOROOT/bin/go-oryx -c conf/oryx.json".

I tried on several VPSes , former one can not work, but later one can work well.

[winlinvip]

Seems the GOROOT is the root of golang, while the GOPATH is the user library. I think user who follow the guide of golang should set the GOPATH correctly.