Schema: update readme for how to validate manually

The latest go is using gomod, but the repo has no go.mod,
adding GO111MODULE=off to disable gomod and build it locally.

Signed-off-by: bin liu <liubin0329@gmail.com>

schema/README.md

@@ -23,7 +23,7 @@ There is also included a simple utility for facilitating validation.
 To build it:
 
 ```bash
-export GOPATH=`mktemp -d`
+export GOPATH=`mktemp -d` GO111MODULE=off
 go get -d ./...
 go build ./validate.go
 rm -rf $GOPATH