Add info about setting default to the README.
Add info about setting default to the README.

Summary:
I don't think it's currently worth automating
the set default processes, so I'm documenting
how to do it manually in the README.

Test Plan: None

Reviewers: csilvers, drosile

Reviewed By: csilvers, drosile

Differential Revision: https://phabricator.khanacademy.org/D40301

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