Move Diffusion ReadmeView to PHUIDocumentProView

Summary: Swapping out to PHUIDocumentProView to remove all calls to PHUIDocumentView.

Test Plan: Review the Phabricator Readme.MD in Diffusion

Reviewers: epriestley, avivey

Reviewed By: avivey

Subscribers: avivey, Korvin

Differential Revision: https://secure.phabricator.com/D15308