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.

Makefile

@@ -1,29 +1,29 @@
-.PHONY: all build install doc clean sdist gf
+.PHONY: all build install doc clean gf # sdist
 
 all: build
 
-dist/setup-config: gf.cabal
-	chmod u+x lib/src/mkPresent
-	chmod u+x lib/src/mkMinimal
-	runghc Setup.hs configure
+dist/setup-config: gf.cabal Setup.hs
+	cabal configure
 
 build: dist/setup-config
-	runghc Setup.hs build
+	cabal build
 
 install:
-	runghc Setup.hs install
+	cabal copy
+	cabal register
 
 doc:
-	runghc Setup.hs haddock
+	cabal haddock
 
 clean:
-	runghc Setup.hs clean
+	cabal clean
+
+#sdist:
+#	cabal sdist
 
-sdist:
-	runghc Setup.hs sdist
 gf:
-	runghc Setup.hs build rgl-none
+	cabal build rgl-none
 	strip dist/build/gf/gf
 
 html::
-	bash bin/update_html
+	bash bin/update_html