docs(README.md): add gitter badge and CDN info to README.md
docs(README.md): add gitter badge and CDN info to README.md

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