Add a note explaining why kerberos is a dependency

[cesarandreu]

It seemed like an unused dependency, and when I searched the repo I couldn't find any mention of what it was for. After googling it, I found out it was apparently required by mongodb for some systems?

I think making a note of that in the structured data tutorial would be helpful.

[ace-n]

I don't think this needs a separate note (outside of perhaps package.json), and adding one would detract from the conciseness of the tutorial. (Also, including the dependency in package.json means that no additional effort is required on the user's part - they have to type npm install anyway, whether this note/dependency is there or not.)

You're correct that Kerberos is sometimes required by MongoDB - more information about that here.