Client session key in an environment variable

This page should be expanded. For now:

Included in Yesod Core, Link to Haddocks

[That link is dead as of 30 October 2015. So how do we put the session key in an environment variable? —Marnen Laibow-Koser]

[The yesod book, specifically the deployment section, links here. Is the correct content available somewhere? -André Cruz]