Update README.md

[yanshiyason]

Add instructions for environment variables.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[coveralls]

Coverage remained the same at 97.167% when pulling cacc814 on yanshiyason:patch-1 into 18611aa on google:master.

[coveralls]

Coverage remained the same at 97.167% when pulling cacc814 on yanshiyason:patch-1 into 18611aa on google:master.

[coveralls]

Coverage remained the same at 97.167% when pulling cacc814 on yanshiyason:patch-1 into 18611aa on google:master.

[yanshiyason]

I signed it!

[googlebot]

CLAs look good, thanks!

[JustinBeckwith]

@TheRoyalTnetennba please take a look, make sure this all looks right, and merge if it's good :)