Add temporary disclaimer

So any internet travelers who stumble upon opendisclosure.io don't think
too much of the numbers until we're done!
Improve templating with content_for

It's one thing that is awesome in Rails templating - the ability to
shove arbitrary content around with content_for. Here, it will allow us
to DRY up our templates between the homepage and other app pages.
Improve README with foreman

Foreman can set the environment variable for DATABASE_URL automatically.
Huzzah!