Update README.md

Author: oehmke

README.md

@@ -1,7 +1,14 @@
 # test_xgrid
 
-  This repository contains test code to allow the comparison of the GFDL
-exchange grid and the ESMF exchange grid.
+  This repository contains test code to allow the comparison of the Geophysical Fluid Dynamics Laboratory (GFDL)
+exchange grid and the Earth System Modeling Framework (ESMF) exchange grid. It creates an exchange grid using either 
+GFDL or ESMF exchange grid creation routines. The exchange grid is created between a cubed sphere atmosphere grid (C48)
+on one side, and a cubed sphere land grid (C48) plus a tripolar ocean grid (720x480) on the other. To do the comparison,
+data is transferred from the atmosphere grid, to the exchange grid, to the land/ocean grids, and then back again. 
+The two types of exchange grid are compared both by how accurately the integral of the data is preserved (conservation)
+between the different stages, and by the time taken to transfer the data between the different stages. The eventual goal is 
+for ESMF's exchange grid to be able to replicate GFDL's closely enough that it can be used to provide functionality like that
+used to couple models in GFDL's Flexible Modeling System (FMS) in ESMF based general purpose model coupling systems. 
 
   To compile, run the compile.csh script in the scripts directory. When compiling on a
 different machine that Hera, a new env file will need to be written to