Add readme on how to generate the docs locally

[ruudk]

I cannot generate the docs locally:

$ make html
sphinx-build -b html -d _build/doctrees   . _build/html
make: sphinx-build: No such file or directory
make: *** [html] Error 1

[cmodijk]

I used $ make autobuild in the past but my machine is broken at the moment will look into it this weekend.

[cmodijk]

Fixed my machine and $ make html and $ make autobuild both work did you install the pip install -r requirements.txt ?

[ruudk]

Yep I did :(