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 :( 
I will probably reduce the required packages to only the docs related ones at some point. If you would like to clean that up in a later PR, that would also be welcome.