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 :( 
'go into the docs folder' repeats the content of the command, this is OK, as long as you interleave the commands with the prose.