Improve SBOM file requirements and instructions in README

[ashsong-nv]

Currently the README does not have enough information on the required SBOM file format and path requirements, and how to generate the SBOM file. This issue is for updating the README with these instructions.

The README should cover these questions:

• How do you generate an SBOM file?
• What is the expected format of the SBOM file?
• Where should the SBOM file be placed? What should the file path be?
• How do you reference an existing SBOM file located in a cloned source repo?
• How do you convert an existing SBOM file in a different format (e.g. directly exported from GitHub) to the expected format?
• We should incorporate or point to this existing README in the data/sboms directory