go/types: add link to tutorial

Change-Id: Ic4f4bc7ea7478908716b951815280e394c55310b
Reviewed-on: https://go-review.googlesource.com/17975
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/api.go

@@ -22,6 +22,8 @@
 // and checks for compliance with the language specification.
 // Use Info.Types[expr].Type for the results of type inference.
 //
+// For a tutorial, see https://golang.org/s/types-tutorial.
+//
 package types // import "go/types"
 
 import (