Add CocoaPod badges.

README.md

@@ -1,3 +1,6 @@
+[![Version](https://img.shields.io/cocoapods/v/GoogleSignIn.svg?style=flat)](https://cocoapods.org/pods/GoogleSignIn)
+[![Platform](https://img.shields.io/cocoapods/p/GoogleSignIn.svg?style=flat)](https://cocoapods.org/pods/GoogleSignIn)
+[![License](https://img.shields.io/cocoapods/l/GoogleSignIn.svg?style=flat)](https://cocoapods.org/pods/GoogleSignIn)
 [![tests](https://github.com/google/GoogleSignIn-iOS/actions/workflows/tests.yml/badge.svg?event=push)](https://github.com/google/GoogleSignIn-iOS/actions/workflows/tests.yml)
 
 # Google Sign-In for iOS