README.mkd: Multiple changes to README

* [General Documentation Change] Add section for required third-party software and Python modules.

* [Critical - Provided Conf Change] Set `check_spam` to 'false' due to observed issues with the Mollom code here, in that the Mollom module no longer has a `MollomAPI` procedure, and keeping `check_spam` as `True` will prevent the application from running.

* [Suggested - Provided Conf Change] Set `relay_enabled` to `False` by default in the example config - this is done because it might not be a good idea to turn it on by default until the configuration has been set up and tested.