node: refine TC percentage rules - CONTRIBUTING.md

Clarifies a more definite two-thirds Quorum.
Formally allows 2 out of 6 members of the TC to be associated with a
single employer.

Currently Stongloop actually holds a 1/3 (33..%) share of the TC, so this formally allows that in writing.

Also it clarifies the majority to be a more definitive 2/3 to be inline with the new TC share. 
(Feel free to dump this part if you guys don't feel it.)