Added README with installation instructions
Update README.rst

Fixed a typo in the README
Added lua:attribute class field declaration, and lua:attr cross-reference.
Merge branch 'classattr'
Added the lua:attribute definition and lua:attr cross reference definition.
These essentially copy the Sphinx py:attribute and py:attr behaviour, since the code itself was taken from sphinx-1.1.3\sphinx\domains\python.py.
Much of the code for the lua domain is similar to the python domain, which is fortunate as I don't fully understand the Sphinx extension API here.

See:
http://sphinx-doc.org/domains.html#directive-py:attribute
http://sphinx-doc.org/domains.html#role-py:attr