doc: reference the readme

Author: julienschmidt

driver.go

@@ -13,6 +13,8 @@
 //  import _ "github.com/go-sql-driver/mysql"
 //
 //  db, err := sql.Open("mysql", "user:password@/dbname")
+//
+// See https://github.com/go-sql-driver/mysql#usage for details
 package mysql
 
 import (