Add documentation for `make pdf-lang`

Contributing.md

@@ -7,3 +7,25 @@
 
 - After you're done with your edits, use `make` to build the pdf and
   verify that everything looks nice.
+
+## Building pdf files
+
+You can build other pdf version of the guide for languages other than
+english. For instance, to build the french version:
+
+    make pdf-lang GUIDE_LANG=fr
+
+This will output a `tutorial-fr.pdf` file.
+
+Currently the repository provides a `guide-*.md` file for the following
+languages:
+
+ Language   | Parameter
+ ---------: | -------------
+ German     | `GUIDE_LANG=de`
+ Greek      | `GUIDE_LANG=el`
+ Spanish    | `GUIDE_LANG=es`
+ French     | `GUIDE_LANG=fr`
+ Italian    | `GUIDE_LANG=it`
+ Portuguese | `GUIDE_LANG=pt`
+ Turkish    | `GUIDE_LANG=tr`