Add custom description to Auspice dataset

[victorlin]

Auspice is currently injecting this generic description below the visualization panels:

This work is made possible by the open sharing of genetic data by research groups from all over the world. We gratefully acknowledge their contributions.

This behavior is slated for removal: https://github.com/nextstrain/auspice/issues/1809

Solution

Create a custom description.md and use it for augur export v2 --description. For inspiration on what to include, see this issue for links to existing descriptions.

[genehack]

Assigning to @kimandrews, to handle as part of #15 .