Update Spreadsheet section in main README.md

- Bring the "Spreadsheet" section of the parent/root directory's README.md up to date with the NDA to provide a more efficient way to download the spreadsheet
- Add "Estimated Time" (of download) and "Estimated Sizes" (of the fast-track-QC spreadsheet, so the user knows when the download is complete)
- Add the number of subjects (overall, per modality, etc.)