net: link skipped TestReadFromUDP to the issue

LGTM=minux
R=bradfitz, minux
CC=golang-codereviews
https://golang.org/cl/154220043

src/net/udp_test.go

@@ -38,7 +38,7 @@ func TestResolveUDPAddr(t *testing.T) {
 func TestReadFromUDP(t *testing.T) {
 	switch runtime.GOOS {
 	case "nacl", "plan9":
-		t.Skipf("skipping test on %q", runtime.GOOS)
+		t.Skipf("skipping test on %q, see issue 8916", runtime.GOOS)
 	}
 
 	ra, err := ResolveUDPAddr("udp", "127.0.0.1:7")