add Makefile for design documents

design/Makefile

+docx:
+	@pandoc -f markdown -t docx -o design.docx design.md
+
+wiki:
+	@pandoc -f markdown -t mediawiki -o design.wiki design.md
+
+all: docx wiki
+
+clean:
+	rm -f design.docx design.wiki
+
+.PHONY: docx wiki clean