Add example for a source level SBOM which includes a subdirectory with a different license

[goneall]

Raised as a question on the SPDX tech email list from Oliver Fendt:

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Sometimes you find in a package (lets assume the declared license is A) in some subdirectory (lets assume SUB-DIR-X) a README file with the following wording: “the files in this directory are licensed under license C”, but none of the files in this directory contain any license information and do not contain any reference to the README file in this directory.
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Response in the email thread:

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In SPDX, you can use the SPDX Package to represent a subdirectory of files within a larger package.

In [the above] scenario, one approach would be to create an SPDX package – if it doesn’t have a logical name, you could call it something like “C licensed files” with a declared license of C. Then use the “contains” relationship from the parent package to the subdirectory.
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