Social media marketing activity tracking - add to README

[RickPack]

Add social media marketing activity tracking to README - plot against registrations. At least use of plus-sign suffixed bit.ly URLs would be interesting:
http://bit.ly/AF2019Signup+
http://bit.ly/AF2019signup+

Also could explore use of #AnalyticsForward hashtag on Twitter, Facebook, and LinkedIn. There's also this NextDoor event URL: https://nextdoor.com/events/nc/durham/analyticsforward-2019-in-durham-by-research-triangle-analysts-2716536