Exercises from the Software Carpentry 4.0 website

Here are a bunch of exercises, markdown-ized, that we used to have as part of the website for various topics.

I wasn't sure where to put them exactly, so I've created a new top-level folder for them, and placed them there, flatly.  I've named things so that the assets for a page of exercises have the same file name prefix (e.g. `vc.markdown`, and `vc_planets.txt`). 

Thoughts?