[WEB-572] Update readme
[WEB-572] Update readme

Test Plan: - read the readme

Reviewers: kevinb, john

Reviewed By: kevinb

Subscribers: #frontend-infrastructure, #webpack

Differential Revision: https://phabricator.khanacademy.org/D52052

Khan/react-render-server,Khan/react-render-server,Khan/react-render-server