the rules for adding Rules

I've added some guidelines for what constitutes good Rule-writing to the wiki at: https://fedorahosted.org/scap-security-guide/wiki/newxccdf (This is also linked from the main page in "How to Create a New Guidance Item in XCCDF"). TLDR? Perhaps. But there are a few recent commits that provided excellent content but which I may reformat to be consistent with these guidelines.