Add a note about blank receiver IDs
Add a note about blank receiver IDs

Change-Id: I3579c8faaa324c0d0915142d58f7b887d3fd956e

tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player