add obligatory xkcd recipe to makefile

Makefile

@@ -86,6 +86,9 @@ example/example.tar.gz: $(example)
 
 # Generic Recipes
 
+presentation/xkcd_%.png:
+	curl -sLo $@ https://imgs.xkcd.com/comics/$*.png
+
 %.tex: %.md
 	pandoc --biblatex -s -o $@ -i $<