Add authorship clarification to README.
Add authorship clarification to README.

jaguerra/showdown,iliakan/showdown,NichenLg/showdown,coolroad/showdown,twitwi/showdown,phw/showdown,mickelfeng/showdown,sahat/showdown,showdownjs/showdown,snegovick/showdown,yaniswang/showdown,rollokb/showdown,webutilz/showdown,jaguerra/showdown,webutilz/showdown,lhagan/showdown,NichenLg/showdown,nilbus/showdown,pazof/showdown,DataDog/showdown,ErisDS/showdown,iliakan/showdown,bsnizek/showdown,lhagan/showdown,showdownjs/showdown,phw/showdown,juanonsoftware/showdown,jaguerra/showdown,twitwi/showdown,stevemao/showdown,bsansouci/showdown,axter/showdown,DataDog/showdown,OMGgogong/showdown,snegovick/showdown,bsnizek/showdown,pazof/showdown,sahat/showdown,xinshitn/showdown,juanonsoftware/showdown,yaniswang/showdown,alazyer/showdown,digideskio/showdown,ErisDS/showdown,rollokb/showdown,mickelfeng/showdown,axter/showdown,OMGgogong/showdown,coolroad/showdown,xinshitn/showdown,nilbus/showdown,alazyer/showdown,digideskio/showdown,ErisDS/showdown,rollokb/showdown,stevemao/showdown,bsansouci/showdown