[skip ci] Update contribution guide
[skip ci] Update contribution guide
Ex-ante-Labs/pivot-ahead,StartPolymer/polymer-starter-kit-plus,anthonybrown/polymer-starter-kit,StartPolymer/polymer-starter-kit,StartPolymer/polymer-starter-kit,StartPolymer/polymer-starter-kit-plus,anthonybrown/polymer-starter-kit,Serious-Rage/poligomy,Ex-ante-Labs/shopper.ninja,abdonrd/polymer-starter-kit,Ex-ante-Labs/shopper.ninja,abdonrd/polymer-starter-kit,Ex-ante-Labs/pivot-ahead,Serious-Rage/poligomy,Serious-Rage/poligomy