Add PDF instructions.

README.md

@@ -24,4 +24,8 @@ and build the Learning VuFind Book
 
 When build process is finished you will find the built html-files in `./build/html`. To view the built documentation open `./build/html/index.html`.
 
+If you prefer a PDF version, you can instead run
+
+    $ make latexpdf
+
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