Updated the README with links to ECMWF's GRIB overviews

README.md

@@ -53,6 +53,14 @@ $ pip install pupygrib[dev,test]
 
 ## Usage
 
+To use pupygrib, you will need a good understanding of the GRIB
+format, especially since table lookups are not yet implemented.  ECMWF
+provides an overview of GRIB
+[edition 1](http://apps.ecmwf.int/codes/grib/format/grib1/overview)
+and
+[edition 2](http://apps.ecmwf.int/codes/grib/format/grib2/overview)
+that can be used as references.
+
 Iterate over the messages in a GRIB file and extract the coordinates
 and values: