Add sites to readme, add mentors list

Update the readme to include links to our NodeSchool site and Meetup
site, and reduce the information contained within it (since the
information is on those sites).  Also include a mentors list which
folks can add to as the group grows in size.

(Most of this format was copied from the London NodeSchool repo.)
Update readme/mentors

Add twitter handles for Enrique and Kearney in the readme, and add them
to the mentors list.