Finish the wiki page about Postgres and Yesod

[begriffs]

https://github.com/begriffs/heroku-buildpack-ghc/wiki/Deploying-Yesod-with-Postgres-or-MySQL

Make a short guide about configuring Yesod and hooking it up to the db using environment variables. Once it's good we'll need a link to it on the README as well.

[begriffs]

There are some docs here to crib from

• http://blog.begriffs.com/2013/08/deploying-yesod-to-heroku-with-postgres.html
• http://pbrisbin.com/posts/parsing_database_url/