add error policy to CONTRIBUTING.md
add error policy to CONTRIBUTING.md

mick-d/nipype,mick-d/nipype,carolFrohlich/nipype,mick-d/nipype,carolFrohlich/nipype,FCP-INDI/nipype,FCP-INDI/nipype,carolFrohlich/nipype,carolFrohlich/nipype,mick-d/nipype,FCP-INDI/nipype,FCP-INDI/nipype