Summary:
The old draft.js Entity API was set to be removed in `v0.11.0`, but that didn't happen so it will now occur in `v0.12.0`. This PR updates the README and website to reflect this.

Pull Request resolved: https://github.com/facebook/draft-js/pull/2651

Differential Revision: D24200501

Pulled By: mrkev

fbshipit-source-id: 7063179ee2f758584d4be2413db923118a67308f