Added the github pages demo to the readme
Added the github pages demo to the readme

Test Plan:
visit https://github.com/Khan/perseus after pushing

alexristich/perseus,huangins/perseus,iamchenxin/perseus,learningequality/perseus,ariabuckles/perseus,junyiacademy/perseus,daukantas/perseus,sachgits/perseus,chirilo/perseus,chirilo/perseus,daukantas/perseus,iamchenxin/perseus,alexristich/perseus,ariabuckles/perseus,abe4mvp/perseus,abe4mvp/perseus,junyiacademy/perseus,ariabuckles/perseus,sachgits/perseus,huangins/perseus,ariabuckles/perseus,learningequality/perseus