clarify checkout goals

it would be helpful to clarify that the goal of the checkout procedures is to familiarize new instructors with each of the key ways to contribute to the Carpentries community. just a small note on the checkout instructions page would do!

Start at the checkout instructions page mentioned in the issue and locate the section describing checkout procedures. Add a brief note explaining that checkout familiarizes new instructors with key ways to contribute to the Carpentries community; the page should clearly state that goal when the work is done.

Written by the indexing model from the issue text.