Add Cocoapods README badge
Add Cocoapods README badge

Summary: Pull Request resolved: https://github.com/facebook/flipper/pull/434

Reviewed By: jknoxville

Differential Revision: D15238279

Pulled By: passy

fbshipit-source-id: a5bd6bf45d0a9cd64d37de9c7c732c6b66403ee8

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