Clarify more information in the README in the ZIPS

[markziemann]

When I downloaded the zip files I could see some information in README.md. It's great that you provide a lot of info here! However, it was unclear to me exactly what the numbers in the file represented technically (e.g. GeneCountMatrix.tsv). i.e. what units are they in? Also, I might have missed this, but what genome/transcriptome have they mapped to?

[markziemann]

Also a minor issue, the README.md has a few trailing spaces, a extraneous newline after # What about metadata, and a trailing newline at the end of the file.