Added section about choosing test variables

I am sure that there are occasions when these libraries would be useful especially when you need larger objects - something like this maybe https://www.npmjs.org/package/factory.js
However, not sure whether they would be in the scope of this PR which is just dealing with choosing variables that could be sensitive. I think that once you know that for emails, do this, passwords, do that etc it is easy enough to not require a library.

On the other hand, being able to call fake_data.password() wherever you need a password does sound pretty cool, but I think we would need to write it.