README: Indicate Important Configs

[parkr]

Indicate what important configs and things to remember for deploying to heroku. Usual gotchas would be helpful, too.

[zeke]

This is pretty vague. Closing.

[parkr]

Configuration options in Rails (like using postgres and database env), using environment variables for things like secret.rb, etc. It's cool I learned on my own how to do this from other places.