Makefile: Add doc target to generate documentation

Makefile

 
-.PHONY: all clean
+.PHONY: all clean doc
 
 all: setup.ml
 	ocaml setup.ml -build
@@ -14,3 +14,6 @@ clean:
 	$(RM) -f _tags
 	$(RM) -f setup.data setup.ml
 	$(RM) -f myocamlbuild.ml
+
+doc: setup.ml
+	ocaml setup.ml -doc