Add open source license

[Zena Lapp]

• Creative Commons license (ideally CC-BY) for text and graphical objects.
• OSI-approved license (MIT?).

[Kelly Sovacool, PhD]

See Carpentries' lessons for a template. We will likely need to use whatever they used since some of our content builds directly on theirs (if they used a copy-left license).
Either way let's be sure to attribute them properly.