Added random generation + text explanation

Wow! Added 500 randomly generated circles and random sizes (radius)
plus some text explanation at the bottom.

Each circle enlarges slowly on click. Still unsure how to manipulate
.data() of circle.canvas properly. Not using the initial randX (random
x coordinates) but NEED to count for number of circles to make? Code
ends up repeating randX and randY within the actual .attr for each
circle WITHOUT ever using the randX and randY data. hmmmmm…