Add repository license (LICENSE file)

[shiv3]

Currently this repository does not include a clear repository-wide license.

Proposed actions:

• Add a LICENSE file at the repository root stating the chosen license (for example: Apache-2.0, MIT).
• Add an SPDX license identifier (for example: "SPDX-License-Identifier: Apache-2.0") to the README and to spec file headers where appropriate.

Purpose:

• So that users and organizations can quickly understand the legal terms for reuse, modification, and distribution.

Request:

• Please decide which license to adopt and add a LICENSE file to the repository root. Also update the README (and spec headers, if applicable) to include the SPDX identifier.

I hope for a license that is easy to use as open-source software.