Add Examples Directory

[JessicaS11]

For new users, it can be unclear what the example notebooks are showing (as noted by @zachghiaccio here). It would be great to add some sort of index with brief descriptions of what each example notebook is showing. Then, either the descriptions or a link to them should be included in the GitHub README.

[zachghiaccio]

Can start on this next week, if nobody else has started working on it.