Add reference

README.md

@@ -25,7 +25,7 @@ geodesic [-i file] [-o file] [-k precision] [-p problem] [-f projection] [-s] [-
 `-f [sphere|ellipsoid|block]` Select projection for evaluating the azimuths and distances.
 * Sphere projection employs the haversine algorithm and spherical triangle formula. Error is higher (error ~0.5%).
 * Ellipsoid projection employs the Vincenty's algorithm with modifications by Charles Karney. It is more resource-intensive (2x sphere projection).
-* Block projection means following parallels of latitude and meridians of longitude. For meridian arcs Bessel's series is implemented. All inputted coordinates should follow meridians of longitude and parallels of latitude.
+* Block projection means following parallels of latitude and meridians of longitude, assuming the Earth is an ellipsoid. For meridian arcs Bessel's series is implemented. All inputted coordinates should follow meridians of longitude and parallels of latitude.
 
 `-s` Different computations for different problems.
 * Distance between coordinates for inverse problems.
@@ -159,3 +159,4 @@ Where
 4. [Determination of areas on the plane, sphere and ellipsoid](https://www.tandfonline.com/doi/abs/10.1179/sre.2006.38.301.583?journalCode=ysre20)
 5. [Geodesics on an ellipsoid of revolution](https://arxiv.org/pdf/1102.1215.pdf)
 6. [Meridian arc @ Wikipedia](https://en.wikipedia.org/wiki/Meridian_arc)
+7. [Longitude @ Wikipedia](https://en.wikipedia.org/wiki/Longitude#Length_of_a_degree_of_longitude)