Document Pillow as a Required Dependency
Document Pillow as a Required Dependency

Summary: Fixes #41

Reviewed By: sjkirby

Differential Revision: D6045309

fbshipit-source-id: fcf0ceffbcdd91d9a4b03a10bad32a0e50588368

facebook/screenshot-tests-for-android,facebook/screenshot-tests-for-android,facebook/screenshot-tests-for-android,facebook/screenshot-tests-for-android