hot fix: fill in READMEs for the Nextclade dataset

[j23414]

Description of proposed changes

Fill in some information for the Nextclade serotype (all) and genotype (denvX) READMEs.

Related issue(s)

• https://github.com/nextstrain/dengue/issues/21

Checklist

• [x] Checks pass

[j23414]

Apparently the README dataset name should match the pathogen.json file based on this comment. Will submit a fix later today