add image at the end of datum section

[ErinBecker]

In this episode, add an image of citrus from https://github.com/MicheleTobias/R-Projections-Workshop at the end of the "Datum" section.

[pmarchand1]

In addition, an image like the one here: http://www.navipedia.net/index.php/Regional_Datums_and_Map_Projections could be helpful to illustrate the local vs. global datum distinction. When describing the datum in the text, should we specifically mention it is a spheroid or ellipsoid approximation to the irregular surface of the Earth?