Add REUSE compliance

[crawfxrd]

REUSE is an FSFE initiative for normalizing the licensing information in a project, using the Linux Foundation's SPDX tags as a base.

The reuse tool replaces the manual grep for the SPDX license tag at the top files, while also adding the following requirements:

• Copyright info must be declared
• Licenses must be valid SPDX IDs
• License texts must included in LICENSES/

Link: https://reuse.software/