add zenodo stats somewhere

They're easy to get from the API so we might as well collect them

```
$ curl -sS https://zenodo.org/api/records/7622545 | jq .stats
{
"downloads": 173.0,
"unique_downloads": 40.0,
"unique_views": 42.0,
"version_downloads": 173.0,
"version_unique_downloads": 40.0,
"version_unique_views": 42.0,
"version_views": 51.0,
"version_volume": 47395962369.0,
"views": 51.0,
"volume": 47395962369.0
}
```