[firebase_analytics] Missing documentation for specifying to add iAd framework to have conversion events tracked correctly in iOS.

[catacs]

In our app we have been using campaign attribution to conversions events since the beginning of publication. We had an important problem with ios campaign attribution. We didn't know why that happened because in the firebase_analytics plugin doesn't specify anything about this. After reviewing the firebase documentation about attribution I saw that we didn't add the iAd framework in ios.
I think it's a good idea to add into readme a disclaimer specifying to add iAd framework to have conversion events tracked correctly in ios.

Debug view without iAd framework in ios:

Debug view with iAd framework in ios:

I realised that at this page: https://support.google.com/firebase/answer/6317518?hl=en