update contributing.md
update contributing.md

ngokevin/aframe,jzitelli/aframe,chenzlabs/aframe,aframevr/aframe,mkungla/aframe,RSpace/aframe,mkungla/aframe,darkwing/aframe,MatthewShotton/aframe,weddingdj/aframe,googlecreativelab/aframe,wizgrav/aframe,jzitelli/aframe,msimpson/aframe,msimpson/aframe,ngokevin/aframe,weddingdj/aframe,jaxmolina/ar,wizgrav/aframe,googlecreativelab/aframe,dmarcos/aframe,darkwing/aframe,aframevr/aframe,RSpace/aframe,jaxmolina/ar,chenzlabs/aframe,fernandojsg/aframe,fernandojsg/aframe,MatthewShotton/aframe,dmarcos/aframe