add some comments

picker_test.go

diff --git a/picker_test.go b/picker_test.go
index 70dae20..0ee789e 100644
--- a/picker_test.go
+++ b/picker_test.go
@@ -184,6 +184,7 @@
 		Op:   naming.Delete,
 		Addr: "127.0.0.1:" + servers[0].port,
 	})
+	// Wait until the first reading is done.
 	wg.Wait()
 	r.w.inject(updates)
 	ctx, _ := context.WithTimeout(context.Background(), 10*time.Millisecond)