Document monitor and ignore lists

[tidoust]

Via #1257. The contributing guidelines attempt to explain how to add a spec to the main list, but nothing documents the monitor and ignore lists. At a minimum, there should be some text somewhere that explains that a spec cannot appear in more than one of these lists, and that the monitor list contains specs and repos that we want to track for periodic review and addition to the main list later on, while the ignore list contains specs and repos we reviewed and decided not to include in the main list.