Fill out README.rst with instructions to build docs and pointer to online version. Refs issue #1

Let me know if I should change something. Also, advices are welcome concerning the built of pdf documentation. I still have some strange error after the build, and the result is a pdf file with basically 6 blank pages :( 
Reduce the required packages to only the docs related one. So you are talking about all the packages installed after a "pip install sphinx", basically: Jinja2==2.7.3
MarkupSafe==0.23, Pygments==2.0.1, Sphinx==1.2.3, argparse==1.2.1, docutils==0.12, wsgiref==0.1.2 Ok, no problem.