databases -> data 

A few thoughts about this introduction, as we iterate it: 

- given how our drivers work, actual users don't actually think about authenticating to the secondaries, so I fear that the last section just introduces more confusion than it clears up. 

- Can we build, someway on whatever text we have for enabling authentication and deploying a replic aset? I feel like the story here is "deploying a replica set is easy, and authentication does this thing, but together you have an extra step (keyfile/etc)"

- It might be nice to have some description of the end goal of the tutorial here, as well, so people can decide if this is the right tutorial for them..