Update ads/README.md to mention the container div. (#7090)
Update ads/README.md to mention the container div. (#7090)

* Update ads/README.md to mention the container div.

* improve wording

rlugojr/amphtml,smartadserver/amphtml,prateekbh/amphtml,Mobusi-ads/amphtml,yandex-pcode/amphtml,mhc-gh/amphtml,kamtschatka/amphtml,comScore/amphtml,camelburrito/amphtml,lannka/amphtml,rajkumarsrk/amphtml,yandex-pcode/amphtml,alanorozco/amphtml,sebastianbenz/amphtml,techhtml/amphtml,engtat/amphtml,koshka/amphtml,yieldbot/amphtml,BenDz/amphtml,chaveznvg/amphtml,sklobovskaya/amphtml,ampproject/amphtml,adiant/amphtml,adiant/amphtml,erwinmombay/amphtml,yieldbot/amphtml,aghassemi/amphtml,keithwrightbos/amphtml,yieldmo/amphtml,wjfang/amphtml,laterpay/amphtml,wjfang/amphtml,fullspeed-sp/amphtml,fullspeed-sp/amphtml,pietergreyling/amphtml,comScore/amphtml,gumgum/amphtml,cory-work/amphtml,adup-tech/amphtml,luciancrasovan/amphtml,zhouyx/amphtml,donttrustthisbot/amphtml,ampproject/amphtml,alanorozco/amphtml,pietergreyling/amphtml,Adtoma/amphtml,kixer-com/amphtml,Gregable/amphtml,widespace-os/amphtml,lannka/amphtml,shawnbuso/amphtml,DAC-f1h-dev/amphtml,arealit/amphtml,avimehta/amphtml,rajkumarsrk/amphtml,ecoron/amphtml,ampproject/amphtml,gumgum/amphtml,zhangsu/amphtml,jridgewell/amphtml,taboola/amphtml,alanorozco/amphtml,pedjoni8/amphtml,alanorozco/amphtml,rsimha-amp/amphtml,nexxtv/amphtml,smartadserver/amphtml,tlong2/amphtml,Mobusi-ads/amphtml,kmh287/amphtml,fullspeed-sp/amphtml,jpettitt/amphtml,Adtoma/amphtml,rsimha-amp/amphtml,JuliusThms/amphtml,ADITION/amphtml,rajkumarsrk/amphtml,tlong2/amphtml,bpaduch/amphtml,kixer-com/amphtml,powdercloud/amphtml,src-code/amphtml,aghassemi/amphtml,src-code/amphtml,honeybadgerdontcare/amphtml,keithwrightbos/amphtml,DistroScale/amphtml,sebastianbenz/amphtml,rsimha-amp/amphtml,keithwrightbos/amphtml,donttrustthisbot/amphtml,danielrozenberg/amphtml,vmarteev/amphtml,kamtschatka/amphtml,adiant/amphtml,erwinmombay/amphtml,donttrustthisbot/amphtml,voyagegroup/amphtml,engtat/amphtml,spotxchange/amphtml,adiant/amphtml,ecoron/amphtml,prateekbh/amphtml,laterpay/amphtml,pietergreyling/amphtml,sharethrough/amphtml,jpettitt/amphtml,widespace-os/amphtml,aghassemi/amphtml,kamtschatka/amphtml,sharethrough/amphtml,dotandads/amphtml,sortable/amphtml,DistroScale/amphtml,src-code/amphtml,alanorozco/amphtml,lannka/amphtml,bpaduch/amphtml,techhtml/amphtml,lzanol/amphtml,bpaduch/amphtml,shawnbuso/amphtml,rajkumarsrk/amphtml,arealit/amphtml,lannka/amphtml,cvializ/amphtml,smartadserver/amphtml,sriramkrish85/amphtml,spotxchange/amphtml,gumgum/amphtml,burtcorp/amphtml,powdercloud/amphtml,zhucl/amphtml,Adtoma/amphtml,Gregable/amphtml,rajkumarsrk/amphtml,jpettitt/amphtml,rajkumarsrk/amphtml,rlugojr/amphtml,sebastianbenz/amphtml,widespace-os/amphtml,techhtml/amphtml,media-net/amphtml,avimehta/amphtml,fullspeed-sp/amphtml,prateekbh/amphtml,keithwrightbos/amphtml,jridgewell/amphtml,lzanol/amphtml,clawr/amphtml,sortable/amphtml,laterpay/amphtml,koshka/amphtml,comScore/amphtml,Adtoma/amphtml,voyagegroup/amphtml,BenDz/amphtml,zhucl/amphtml,media-net/amphtml,clawr/amphtml,camelburrito/amphtml,cvializ/amphtml,shawnbuso/amphtml,honeybadgerdontcare/amphtml,zhucl/amphtml,dotandads/amphtml,pedjoni8/amphtml,mhc-gh/amphtml,sharethrough/amphtml,lannka/amphtml,mhc-gh/amphtml,engtat/amphtml,keithwrightbos/amphtml,src-code/amphtml,adup-tech/amphtml,kmh287/amphtml,voyagegroup/amphtml,jpettitt/amphtml,sortable/amphtml,cory-work/amphtml,danielrozenberg/amphtml,ADITION/amphtml,kamtschatka/amphtml,wjfang/amphtml,danielrozenberg/amphtml,kmh287/amphtml,smartadserver/amphtml,rlugojr/amphtml,engtat/amphtml,jridgewell/amphtml,tlong2/amphtml,JuliusThms/amphtml,aghassemi/amphtml,donttrustthisbot/amphtml,adiant/amphtml,media-net/amphtml,rajkumarsrk/amphtml,Mobusi-ads/amphtml,comScore/amphtml,jpettitt/amphtml,spotxchange/amphtml,Mobusi-ads/amphtml,dotandads/amphtml,arealit/amphtml,sebastianbenz/amphtml,media-net/amphtml,jmadler/amphtml,ADITION/amphtml,luciancrasovan/amphtml,rsimha-amp/amphtml,honeybadgerdontcare/amphtml,laterpay/amphtml,danielrozenberg/amphtml,Gregable/amphtml,media-net/amphtml,camelburrito/amphtml,bpaduch/amphtml,jpettitt/amphtml,jmadler/amphtml,camelburrito/amphtml,kixer-com/amphtml,shawnbuso/amphtml,aghassemi/amphtml,techhtml/amphtml,danielrozenberg/amphtml,lannka/amphtml,laterpay/amphtml,erwinmombay/amphtml,yieldbot/amphtml,burtcorp/amphtml,jdelhommeau/amphtml,prateekbh/amphtml,engtat/amphtml,koshka/amphtml,bpaduch/amphtml,widespace-os/amphtml,Adtoma/amphtml,yieldmo/amphtml,jpettitt/amphtml,kixer-com/amphtml,smartadserver/amphtml,gumgum/amphtml,engtat/amphtml,mhc-gh/amphtml,zhangsu/amphtml,prateekbh/amphtml,lzanol/amphtml,jdelhommeau/amphtml,rlugojr/amphtml,powdercloud/amphtml,JuliusThms/amphtml,koshka/amphtml,ADITION/amphtml,tlong2/amphtml,donttrustthisbot/amphtml,lzanol/amphtml,nexxtv/amphtml,jmadler/amphtml,rlugojr/amphtml,taboola/amphtml,widespace-os/amphtml,Gregable/amphtml,vmarteev/amphtml,aghassemi/amphtml,DAC-f1h-dev/amphtml,comScore/amphtml,jridgewell/amphtml,yandex-pcode/amphtml,DistroScale/amphtml,sklobovskaya/amphtml,dotandads/amphtml,chaveznvg/amphtml,honeybadgerdontcare/amphtml,avimehta/amphtml,sortable/ampht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