README.rst See also SPDX-license-expressions link 404 for old appendix-IV

[BrianInglis]

See also link in README.rst to SPDX License Expressions gives 404 from:

See also for details:
- https://spdx.github.io/spdx-spec/appendix-IV-SPDX-license-expressions/

that info is now at:
SPDX / annexes / SPDX License Expressions
Annex D: SPDX license expressions (Normative)
https://spdx.github.io/spdx-spec/v3.0/annexes/SPDX-license-expressions/
which might be linked as:

[Annex D: SPDX license expressions (Normative)](https://spdx.github.io/spdx-spec/v3.0/annexes/SPDX-license-expressions/)

[pombredanne]

@BrianInglis Thanks!
I guess this is also an issue for SPDX that does not make their URLs permanent
I have yet reviewed the 3.0 spec and we likely do not support it yet fully, FWIW.