License badge in 3D Slicer and related repositories `README` files

[jcfr (Jean Christophe Fillion Robin (Kitware))]

To help with this, we discussed submitting the Slicer license so that it is added to the SPDX license list.

This should then allow us to move forward with having the license detected by GitHub, which in turn should allow to automatically generate a badge.

Proposal: Add the Slicer license to SPDX license list

What is the SPDX License List ? The SPDX License List is an integral part of the SPDX Specification. The SPDX License List itself is a list of commonly found licenses and exceptions used in free and open or collaborative software, data, hardware, or documentation. The SPDX License List includes a standardized short identifier, the full name, the license text, and a canonical permanent URL for each license and exception. See SPDX License List | Software Package Data Exchange (SPDX) Why shou…