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 :( 
As you noted, this doesn't currently work. I'll open a separate issue for the problem, and see what I can figure out. Meanwhile lets keep the section.