Add LICENSE file with the project's BSD-3-Clause terms

[parasew]

The source files carry BSD-3-Clause headers, but the repository had no standalone LICENSE file. This adds one reproducing the license text and copyright notice already used in the source headers. It does not change the license.

This helps license scanners, SBOM tooling, and downstream packaging that expects a root-level license file (e.g. OpenWrt's PKG_LICENSE_FILES).

Refs: #76

[parasew]

Adds a root-level LICENSE file, as agreed in #76.
The text is reproduced verbatim from the BSD-3-Clause headers already present in the source (copyright line, all three clauses, disclaimer, and the closing paragraphs). This documents the existing license rather than changing it.
Motivation: OpenWrt's packaging requires a PKG_LICENSE_FILES parameter pointing at an upstream license file, and there was none to reference (openwrt/routing#1176). It also helps license scanners and SBOM tooling generally.
Happy to adjust the wording or placement if the team prefers a different form.