pathogen/files: Add data use language

Add paragraph to top of the page to acknowledge data generators, avoid scooping, and warn about RESTRICTED files. See [zika description](https://github.com/nextstrain/zika/blob/main/phylogenetic/defaults/description.md) and [rsv description](https://github.com/nextstrain/rsv/blob/master/config/description.md) for examples.