Adding info on feature metadata format in README

Hi there,

thanks for developing empress... it's great! However, I can't seem to find how to format the feature metadata tsv file required by parameter --feature-metadata. I imagine that node labels and color code should be included somehow, but just don't know how to do it in the right format.

Would be great to provide this information in README.md, or, at least, include a tsv file with a minimal example.

Thanks!

Start with README.md and locate the implementation or command-line handling for --feature-metadata to confirm the accepted TSV structure. Update the README with the required format and a minimal TSV example, including any supported node-label or color fields, so users can prepare the file without guessing.

Written by the indexing model from the issue text.