more Go Module notes

README.md

@@ -57,7 +57,7 @@ Ok not to use it if your app project is really small and where an extra level of
 
 ### `/vendor`
 
-Application dependencies (managed manually or by your favorite dependency management tool like the new built-in [`Go Modules`](https://github.com/golang/go/wiki/Modules) feature).
+Application dependencies (managed manually or by your favorite dependency management tool like the new built-in [`Go Modules`](https://github.com/golang/go/wiki/Modules) feature). The `go mod vendor` command will create the `/vendor` directory for you. Note that you might need to add the `-mod=vendor` flag to your `go build` command if you are not using Go 1.14 where it's on by default.
 
 Don't commit your application dependencies if you are building a library.