Added makefile target to remove generated docs

Makefile

@@ -26,7 +26,7 @@ OCAMLFLAGS := -g
 OCAMLYACC := menhir
 YFLAGS := --infer --explain
 
-.PHONY: all check dist testsuite myclean
+.PHONY: all check dist testsuite myclean cleaner
 
 all: native-code
 
@@ -58,4 +58,7 @@ $(TESTSUITE_TGT): TESTSUITE_README test
 myclean:
 	rm -f a.out $(DIST_TGT) $(TESTSUITE_TGT)
 
+cleaner: clean
+	rm -rf doc/$(RESULT)
+
 include OCamlMakefile