Update ios.md

[mrlescodes]

Adds additional information to the docs as described in:
https://developers.google.com/ad-manager/mobile-ads-sdk/ios/quick-start

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[mikehardy]

Interesting - this looks great on the face of it (thank you!) but are there any situations it isn't necessary or is this always necessary now? Stated differently, is this only for Ad Manager apps, and it is possible that some app is not an Ad Manager app but still uses AdMob? If there are different classes of app we need to explain under exactly what circumstances this is needed, otherwise it's fine as is