add xkcd example png
add xkcd example png

iamjakob/nbviewer,thomasyu888/nbviewer,Jay-Oh-eN/nbviewer,roxyboy/nbviewer,imsparsh/nbviewer,Jay-Oh-eN/nbviewer,iamjakob/nbviewer,Sage-Bionetworks/nbviewer,christophelec/nbviewer,franblas/beaker-sharing-server,franblas/beaker-sharing-server,twosigma/beaker-sharing-server,franblas/beaker-sharing-server,twosigma/beaker-sharing-server,franblas/beaker-sharing-server,twosigma/beaker-sharing-server,thomasyu888/nbviewer,twosigma/beaker-sharing-server,imsparsh/nbviewer,bollwyvl/nbviewer,Sage-Bionetworks/nbviewer,AlfiyaZi/nbviewer,bollwyvl/nbviewer,roxyboy/nbviewer,christophelec/nbviewer,AlfiyaZi/nbviewer