Add contributing guidelines.
Add contributing guidelines.

bricegnichols/urbansim,AZMAG/urbansim,UDST/urbansim,ual/urbansim,ual/urbansim,waddell/urbansim,apdjustino/urbansim,waddell/urbansim,bricegnichols/urbansim,UDST/urbansim,AZMAG/urbansim,AZMAG/urbansim,VladimirTyrin/urbansim,SANDAG/urbansim,VladimirTyrin/urbansim,apdjustino/urbansim,SANDAG/urbansim,apdjustino/urbansim,bricegnichols/urbansim,bricegnichols/urbansim,VladimirTyrin/urbansim,synthicity/urbansim,UDST/urbansim,waddell/urbansim,SANDAG/urbansim,synthicity/urbansim,apdjustino/urbansim,synthicity/urbansim,VladimirTyrin/urbansim,ual/urbansim,ual/urbansim,synthicity/urbansim,waddell/urbansim,SANDAG/urbansim,AZMAG/urbansim,UDST/urbansim