Makefile: Document how to build

Change-Id: Ief393626f9ca5a838eba574e6b70587405118d4e
Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/Makefile b/Makefile
index 7f6c891..e4c0f52 100644
--- a/Makefile
+++ b/Makefile
@@ -1,3 +1,6 @@
+# To build:
+#  $ apt install fonts-linuxlibertine texlive-xetex pandoc
+#  $ make
 
 # we need xelatex for utf-8 support
 tex = xelatex