Adding a note about not trying to use Python 3.3.x to build docs -
requires Python 2.7. Python 3 support is possible, but sphinx-extensions
needs several changes.

Also adding excludes for a few backup files and such so they don't
get committed to the repo accidentally.
Merge pull request #64 from robertrolandorg/update_readme

Adding a note about not trying to use Python 3.3.x to build docs -