Update readme to warn of deprecation

Author: afonsoafonsoafonso

README.md

@@ -1,4 +1,9 @@
-# Velocidi iOS SDK in Objective-C
+# DEPRECATED iOS SDK in Objective-C
+
+
+This **SDK has been deprecated** and is no longer maintained. We suggest using our [iOS platform guide](https://docs.audience.kevel.com/platform-guides/ios) instead.
+
+[![No Maintenance Intended](http://unmaintained.tech/badge.svg)](http://unmaintained.tech/)
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 [![Carthage compatible](https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat)](https://github.com/Carthage/Carthage)