Add NDA download prep directions to the README.md

[ericearl]

The README currently relies on users to download both a full fast-track collection and the latest abcd_fastqc01.txt file, but it doesn't provide the instructions. The instructions mostly exist here already:

• https://github.com/DCAN-Labs/abcd-dicom2bids?tab=readme-ov-file#package-for-abcd_fastqc01txt
• https://github.com/DCAN-Labs/abcd-dicom2bids?tab=readme-ov-file#package-for-data-image

[ericearl]

Preparation notes have been added to docs/prepare.md