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 :( 
This should probably be a separate venv than the one we're going to use for the tutorial. Perhaps ```morepath_docs_env```, or do you think that's too long?