go.mobile/app: add comment about X11 library dependencies.
go.mobile/app: add comment about X11 library dependencies.

LGTM=adg, crawshaw
R=crawshaw, adg
CC=golang-codereviews
https://golang.org/cl/159530043

xing/gomobile,MaxCy330/mobile,golang/mobile,DanielWorld/mobile,qskycolor/mobile,cainiaocome/mobile,swisspol/mobile,tempbottle/mobile,lonphy23t/mobile,cainiaocome/mobile,DanielWorld/mobile,ppamorim/mobile,steeve/mobile,ppamorim/mobile,0x90sled/mobile,xing/gomobile,samanalysis/mobile,ichu501/mobile,ichu501/mobile,MaxCy330/mobile,cainiaocome/mobile,lonphy23t/mobile,steeve/mobile,swisspol/mobile,goldcaddy77/mobile,goldcaddy77/mobile,0x90sled/mobile,xing/gomobile,steeve/mobile,ppamorim/mobile,zach-klippenstein/mobile,b54898533/mobile,tempbottle/mobile,zach-klippenstein/mobile,b54898533/mobile,zach-klippenstein/mobile,0x90sled/mobile,qskycolor/mobile,samanalysis/mobile,cainiaocome/mobile,samanalysis/mobile,tempbottle/mobile,grategames/mobile,freeman0211/mobile,goldcaddy77/mobile,tempbottle/mobile,lonphy23t/mobile,b54898533/mobile,goldcaddy77/mobile,swisspol/mobile,freeman0211/mobile,grategames/mobile,grategames/mobile,samanalysis/mobile,swisspol/mobile,qskycolor/mobile,ichu501/mobile,golang/mobile,ichu501/mobile,ppamorim/mobile,DanielWorld/mobile,samanalysis/mobile,ichu501/mobile,zach-klippenstein/mobile,freeman0211/mobile,MaxCy330/mobile,lonphy23t/mobile,DanielWorld/mobile,grategames/mobile,cainiaocome/mobile,MaxCy330/mobile,xing/gomobile,qskycolor/mobile,freeman0211/mobile,swisspol/mobile,b54898533/mobile,tempbottle/mobile,golang/mobile,goldcaddy77/mobile,ppamorim/mobile,xing/gomobile,b54898533/mobile,0x90sled/mobile,0x90sled/mobile,MaxCy330/mobile,golang/mobile,lonphy23t/mobile,steeve/mobile