Add a bit more explanation to the extra_packages variable

[sidneygijzen]

Today I ran into the extra_packages variables in my config.yml file. I had a hunch that it could be of use to me, but I had to track it back a bit to provisioning/tasks/extras.yml and to this ticket to understand what it exactly does.

In my case it was perfect to provision the unzip package (for usage in downloading libraries by Drush make files). So, I propose to add a bit more explanation to the variable to make it more clear on how to use it and what it expects.

[geerlingguy]

I like the idea—sometimes it's best to have a little more explicit direction, especially since the name of the variable is generic and doesn't include 'apt' or 'yum' in it. I'm going to merge this work then try to make the documentation line a little more succinct.