Document .env file usage

see also dashboard-backend!73 (comment 46118)

I needed to add this .env file in the dashboard repo before using yarn start:

REACT_APP_API_URL=http://stackspin_proxy:8081/api/v1
REACT_APP_HYDRA_PUBLIC_URL=https://sso.varac-test.stackspin.net

This should be documented in the README.md

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information