Update add_to_app README (#129)

* Update add_to_app README

* simplify README prerequisites

* Cocoapods -> CocoaPods

experimental/add_to_app/README.md

@@ -1,7 +1,8 @@
 # Add-to-App Sample
 
-***The Add-to-App sample is designed to build with Flutter's `master` channel. See
-the [README](../README.md) in the `experimental` directory for details.***
+***The Add-to-App sample is designed to build with Flutter's `master` channel 
+and the latest version of CocoaPods. See the [README](../README.md) in the 
+`experimental` directory for details.***
 
 This directory contains a bunch of Android and iOS projects that each import
 a standalone Flutter module called `example_module`.