* Adds instructions for creating a PDF of the documentation

I believe the docs requirements are correct to get this to work.

i.e. add:

`pip install sphinx-simplepdf`

then

`sphinx-build -b simplepdf  -W -E -T  -a docs /tmp/mydocs`

from the root of the repository. It should take a few minutes
then to build a PDF that will turn up under `/tmp/mydocs`.

* Adds pdf creation to read the docs

Let's see if this works.