"noPeriod" key in data.json ... and a contribution suggestion

[jeffreysbrother]

I see that one site listed in data.json has a "noPeriod" key, but doing a search for "noPeriod" returns zero results. So it seems like Sherlock is not even using this. That's just an observation. Perhaps this is a placeholder for future functionality?

Also, it might be useful to mention the optional --local flag in the CONTRIBUTING file so that a person attempting to add/remove/modify a site will see those changes locally.