Update README.md

Summary:
rnystrom - I know you removed this before, but I think we should add it back now that we support > 1 platform. 😄

(and macOS is coming up in #235 )
Closes #322

Differential Revision: D4314033

Pulled By: rnystrom

fbshipit-source-id: d768d0ce19df0154609ab639f0acb8d95fe2b7da

README.md

@@ -15,6 +15,10 @@
         <img src="https://img.shields.io/cocoapods/v/IGListKit.svg?style=flat"
              alt="Pods Version">
     </a>
+    <a href="https://instagram.github.io/IGListKit/">
+        <img src="https://img.shields.io/cocoapods/p/IGListKit.svg?style=flat"
+             alt="Platforms">
+    </a>
     <a href="https://github.com/Carthage/Carthage">
         <img src="https://img.shields.io/badge/Carthage-compatible-brightgreen.svg?style=flat"
              alt="Carthage Compatible">