developer_guide: mention explicitly the GOPATH dir
@@ -22,6 +22,7 @@ Required tools:
Build in project root dir:
```
+( under $GOPATH/src/github.com/coreos/etcd-operator/ )
$ IMAGE=${your_image} hack/build/operator/build
```
`IMAGE` is the container image, e.g. "gcr.io/coreos/etcd-operator" .
