Explain the namespace use in SPDX 3.0 for custom licenses

[silverhook]

Would probably fit in FAQ, and is pending spdx/spdx-spec#113 and SPDX Spec 3.0 to release, but wanted to include it here for tracking purposes.

This change would avoid potential clashes between two people naming their licenses the same way, while the content is different, because a new custom license’s SPDX ID would include the namespace of the project or company that wrote it.

[mxmehl]

Thanks! Indeed, the FAQ seems to be the most appropriate place to add this info. I don't see it touching the REUSE spec since it's under the License-Ref hood.