Mention Haskell EDSL implementation in main readme.

Author: eldering

README.md

@@ -16,6 +16,10 @@ according to which the testdata is checked.  Two sample scripts
 
 [Checktestdata language specification](doc/format-spec.md)
 
+We also have a [Haskell implementation](haskell_edsl/README.md) of the
+checktestdata program, which supports this specification as well as a
+Haskell embedded domain-specific language.
+
 ## Installation