[WIP] ERD (Entity Relationship Diagram)

An early step in development is defining the data the application will consume.  This PR will track the development of the ERD and is not intended for merge until a consensus is reached.

Version 1 of this document includes a Member entity formatted for zfc-user, a Meetup entity to link into the Meetup API, Presentations with a many to many for Meetup for multiple presentations in a meetup, MeetupUser which will be used to extend the RSVP offered by Meetup to include checkin through the sfphp.org app at the event.  And Sponsor including sponsor contacts and a many to many for each meetup.  

There is a isSiteListed bool on Sponsor which could be folded into a SponsorType entity.

This ERD is created with Skipper www.skipper18.com