Add URL of hello.p5js.org to repo description

[ffd8]

I was trying to track down this great video to share with new students, but couldn't find the video linked on the official p5.js website. Found this repo through search and still couldn't find the hosted URL until digging through the closed issues discussion for potential subdomains. It would be helpful to add the hello.p5js.org to the currently blank top repo description for anyone who stumbles across it in a similar fashion.