add contributing doc [skip ci]
add contributing doc [skip ci]

mojodna/node-pre-gyp,mapbox/node-pre-gyp,mojodna/node-pre-gyp,mapbox/node-pre-gyp,mojodna/node-pre-gyp,mojodna/node-pre-gyp,mapbox/node-pre-gyp,mapbox/node-pre-gyp,mapbox/node-pre-gyp,mojodna/node-pre-gyp