Provide instructions for obtaining Guide as PDF

[Castaglia]

Some folks like being able to get documentation as PDF; thus provide instructions for converting the entire Developer Guide into a single PDF.

This might also include doing the PDF conversion as part of the Travis CI build, and uploading the generated PDF to GitHub somewhere?

[Castaglia]

It looks like the most commonly used tool for this is markdown-pdf; see:

http://hacking.elboulangero.com/2016/07/21/markdown-to-pdf.html