design: add seeking on encoded data as an open question

Change-Id: I561286c38b1c90b4115cae61d0ea0e51904eb460
Reviewed-on: https://go-review.googlesource.com/18170
Reviewed-by: Burcu Dogan <jbd@google.com>
diff --git a/design/13432-mobile-audio.md b/design/13432-mobile-audio.md
index a3da99a..062f473 100644
--- a/design/13432-mobile-audio.md
+++ b/design/13432-mobile-audio.md
@@ -245,6 +245,7 @@
 is primarily mobile devices (which case-by-case supports hardware
 acceleration). But if the package will cover beyond the mobile, we should
 consider this case.
+- Seeking on variable bit rate encoded audio data is hard without a seek table.
 
 ## Appendix: Audio Apps Classification