document files and give instructions for creating signatures for any given prevalence threshold

[lwaldron]

Use GitHub Action to add .gmt files for (1%, 10%, 30%, 50%, 70%, 90% prevalence cutoff thresholds) x (NCBI ID and taxonomic names) for use similarly to https://github.com/waldronlab/BugSigDBExports

[lwaldron]

Actually, it's OK to leave it as just these four files, with documentation in the README.md about what they contain, and code/instructions for producing signatures from them.