Add "clean" rule
diff --git a/docs/architecture/Makefile b/docs/architecture/Makefile
index 5bde640..258abcd 100644
--- a/docs/architecture/Makefile
+++ b/docs/architecture/Makefile
@@ -18,3 +18,6 @@
 	$(PANDOC) -o $@ $<
 .md.pdf:
 	$(PANDOC) -o $@ $<
+
+clean:
+	rm -f *.html *.pdf