Add heroku tip

[thenickcox]

Some students were confused about why the Heroku page didn't have data on it after they deployed. This PR adds a tip to this section explaining why Heroku will have a copy of their code, but not their database.

[thenickcox]

I looked back over this, and it actually says pretty much this same thing in the preceding paragraph, but there were 3-4 students that got stuck on this step, thinking that they had broken something when they opened the app in Heroku. I think it's because they read the code heavily, but not the explanations. Putting this in a tip, rather than a plain text paragraph, makes this much clearer, and it gives students the information right when they're likely to be confused, not before. (It also answers the question, "Why would I want Heroku to have different data than what's on my machine?", which at least one student asked.)