Include documentation of how SPDX files are created

[lee0c]

It would be great to see the specific tooling calls used to create each SBOM in the repo to understand how the different styles and specific sets of info are built out.

Or, if they are created manually, it would be worth specifying that somewhere.

[swinslow]

Thanks @lee0c! Great suggestion.

For examples 1 through 6, for most of the SPDX documents I started by using the Golang SPDX tools builder and idsearcher packages. I then tweaked the results by hand to add extra data to illustrate the particular example.

For example 7, I believe @nishakm created these by hand.

I'll plan to add a PR to clarify this in the repo docs. Thanks for suggesting it!