Add tip that welcome page will not show if using Rails 4.0.x.

Some students using Rails 4.0.x were confused about why the Welcome page showed on their localhost, but not when they pushed to Heroku.
Reword tip that I just added about the default welcome page not showing on Heroku if using Rails 4.
Merge branch 'master' of https://github.com/SamSamskies/docs into SamSamskies-master