add docsclean target

Signed-off-by: martin f. krafft <madduck@madduck.net>
diff --git a/Makefile b/Makefile
index 85ea630..5453180 100644
--- a/Makefile
+++ b/Makefile
@@ -45,3 +45,6 @@
 	  git push $(shell git config --get branch.$(BRANCH).remote) $(BRANCH); \
 	fi
 	git checkout '@{-1}'
+
+docsclean:
+	$(MAKE) -C doc clean