Added job posting guidelines finally

This is "heavily influenced" by the Boston Python groups job posting guidelines: http://www.meetup.com/bostonpython/pages/Job_Posting_requirements/

Any thoughts? I left off the part about no mystery jobs hiding behind a recruiter. Any thoughts on whether we should add that back in?
You can also make this `<PythonSD-list@meetup.com>` I believe, and remove the need for the link below.