add latexmk option for lualatex (closes issue #17)

Author: jeffhammond

README.txt

@@ -5,7 +5,8 @@ The document can be generated using the following command:
 latexmk -pdf -silent cplexts
 
 Note: On some systems (Mac), the previous comment will invoke pdflatex,
-which does not work for this document.  In this case, one needs to
-invoke lualatex directly.
+which does not work for this document.  In this case, build with this command:
+
+latexmk -pdf -lualatex -silent cplexts
 
 Users of Windows should see WINDOWS.txt.