Document the source of the data

[jakobjpeters]

The website should document how the download number is determined (is this just only user downloads, is it requests to Pkg.add, is it GitHub clones?), where the data comes from, and how often the data is updated.

The only information I can find about this (without digging through source code) is this comment. This leads me to believe it's Pkg.add, but I'm not sure. The number without explanation could mean many different things.

[essenciary]

Good point, we can explain the data and the algorithm.