Document codesigning issue for iOS SDK (#430)

Added a section about codesigning issues for iOS SDK v19.5.6 and v19.5.7.

Author: Br1an-Boyle

README.md

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 <img src="https://user-images.githubusercontent.com/6392766/92717163-af7b0200-f357-11ea-9dc3-6f86d0c21d96.png" width="297" height="76"/>
 
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+## ⚠️ May 1st Codesigning issue ⚠️ 
+If you are running iOS SDK [v19.5.6](https://github.com/intercom/intercom-ios/releases/tag/19.5.6) or [v19.5.7](https://github.com/intercom/intercom-ios/releases/tag/19.5.7) and are experiecing a codesigning issue, [please follow these steps](https://github.com/intercom/intercom-ios/wiki/Codesigning-Issue) to resolve it.
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