update readme with vendoring details

[Sheshagiri]

Not a bug as such, it would be nice to add a note to the readme on how to get the go app that is vendored built using a multistage Dockerfile.

The current readme

WORKDIR /go/src/app
COPY main.go .

RUN /usr/local/go/bin/go build -o app .

doesn't seem to work as /go/src/app is not it gopath. Happy to submit a pr with updated readme.

WORKDIR /root/go/src/app
COPY . .

RUN /usr/local/go/bin/go build -o app .

setting the workdir to /root/go/src/app did the magic for me :-) . I guess updating the WORKDIR to /root/go/src/app works for everyone including apps coming with vendor dependencies.