chore: add CONTRIBUTING.md
chore: add CONTRIBUTING.md

juanonsoftware/showdown,phw/showdown,xinshitn/showdown,yaniswang/showdown,axter/showdown,xinshitn/showdown,axter/showdown,phw/showdown,OMGgogong/showdown,bsnizek/showdown,stevemao/showdown,yaniswang/showdown,digideskio/showdown,showdownjs/showdown,NichenLg/showdown,OMGgogong/showdown,bsansouci/showdown,twitwi/showdown,digideskio/showdown,alazyer/showdown,stevemao/showdown,bsansouci/showdown,coolroad/showdown,snegovick/showdown,webutilz/showdown,webutilz/showdown,snegovick/showdown,coolroad/showdown,twitwi/showdown,NichenLg/showdown,showdownjs/showdown,juanonsoftware/showdown,pazof/showdown,alazyer/showdown,bsnizek/showdown,pazof/showdown