Mention flow init when doesn't find .flowconfig

fixes #19.

I'm not sure what the policy is on multiline error messages, but this looked a little better on separate lines IMO.  I tested the change locally.

I'm also not sure if this extra message is needed once people become familiar with the tool (or look at the --help options).  I found out pretty quickly I needed to run `flow init`, so I'm guessing others will as well.  Feel free to ignore/close this pull request.  I figured it was a ticket I could close out quickly (when looking at the issues).