docs: re-add License section to README

[martinydeAI]

Surfaced while doing the #44 ADR-followup cleanup.

ADR 002 (project license: MPL-2.0) lists "Reference the license in
README.md" as a completed follow-up, but the README rewrite from
#28 dropped the License section. README.md on develop currently
has no mention of MPL-2.0 or the LICENSE file.

Re-add a short License section to README.md that:

• States the project is MPL-2.0 licensed.
• Links to the LICENSE file at the repo root.
• Links to docs/adr/002-project-license-mpl-2.md for the decision
  context.

Refs ADR 002, #28, #32.