Updated the GF Developers Guide

+ The instructions have been modernized and simplified slightly and should now
  work again. (I have not verified it on Windows.)
  Using 'cabal' instead of 'runghc Setup.hs' avoids some problems.
+ Also, download/index.html has simpler installation instructions sufficient
  for many users, so mention it in the main README file.
+ Also adjusted the main Makefile.

README

@@ -30,4 +30,5 @@ GF particularly addresses four aspects of grammars:
 
 
 COMPILATION and INSTALLATION of source distribution:
-See doc/gf-developers.html for installation instructions.
+See download/index.html for installation instructions.
+(More details can be found in doc/gf-developers.html.)