Add ring model documentation #53
Description
There is a minimal description of the rings available whitin
https://fermi.gsfc.nasa.gov/ssc/data/analysis/software/aux/4fgl/Galactic_Diffuse_Emission_Model_for_the_4FGL_Catalog_Analysis.pdf
A minima you could specify that these rings are the ones used in the Fermi-LAT diffuse model associated to 4FGL catalogue and add a reference to that page.