Adding a flag for Cocoa Pods users

Using the Parse framework and SDWebImage frameworks installed via Cocoa Pods gives the same error as described in the Alternatively section of the Add Linker Flag section in this Readme.
I found a solution here : http://www.deanmao.com/2012/12/31/linker-error-using-cocoapods/
Merge pull request #879 from dulgan/patch-1

Adding a flag for Cocoa Pods users