Mention download counts being stored

[sigurdm]

[sigurdm]

cc @mit-mit

[jonasfj]

I'd suggest we update this when we remove popularity metrics. In particular, I think it'd make sense to explain how download counts factor into search rankings, and to explain how download counts are useful, and at what version range granularity they are available, and why -- how do these help package author determine the migration of package consumers towards newer versions.