Add nrmysql example link
// ctx := newrelic.NewContext(context.Background(), txn)
// row := db.QueryRowContext(ctx, "SELECT count(*) from tables")
//
+// A working example is shown here:
+// https://github.com/newrelic/go-agent/tree/master/_integrations/nrmysql/example/main.go
package nrmysql
import (
