Update README.md (#2391)

fix: execute gqlgen generate command error.  eg: systems failed: unable to build object definition: unable to find type: github.com/99designs/gqlgen/graphql/introspection.InputValue. need import  github.com/99designs/gqlgen/graphql/introspection .

README.md

@@ -22,7 +22,8 @@ Still not convinced enough to use **gqlgen**? Compare **gqlgen** with other Go g
 
 2. Add `github.com/99designs/gqlgen` to your [project's tools.go](https://github.com/golang/go/wiki/Modules#how-can-i-track-tool-dependencies-for-a-module)
 
-       printf '// +build tools\npackage tools\nimport _ "github.com/99designs/gqlgen"' | gofmt > tools.go
+       printf '// +build tools\npackage tools\nimport (_ "github.com/99designs/gqlgen"\n _ "github.com/99designs/gqlgen/graphql/introspection")' | gofmt > tools.go
+
        go mod tidy
 
 3. Initialise gqlgen config and generate models