Add a vignette

[sckott]

This package needs more documentation! Help out the community by contributing a vignette. If you don't know what a vignette is, check out http://r-pkgs.had.co.nz/vignettes.html for an introduction.

If you aren't sure how to contribute on github checkout https://github.com/ropensci/pangaear/blob/master/.github/CONTRIBUTING.md

Keep in mind our code of conduct https://github.com/ropensci/pangaear/blob/master/CONDUCT.md