Add vndr to go-compile and add vendoring docs

This makes it easier to safely udpate the vendor directory

Signed-off-by: Dave Tucker <dt@docker.com>

tools/go-compile/Dockerfile

@@ -2,6 +2,7 @@ FROM alpine:3.5
 RUN apk update && apk add --no-cache build-base git go
 ENV GOPATH=/go PATH=$PATH:/go/bin
 RUN go get -u github.com/golang/lint/golint
+RUN go get -u github.com/LK4D4/vndr
 
 COPY . ./