Summary:
Add Podfile documentation so that projects that use `use_frameworks!` have a way to import Flipper into their project
Pull Request resolved: https://github.com/facebook/flipper/pull/347
Reviewed By: passy
Differential Revision: D13761700
Pulled By: priteshrnandgaonkar
fbshipit-source-id: 36d8a5d590b96203ac30fd5d0ca32cae4f194024