docs: add Ecosystem / Adopters section with CATAAM

[avinashanand]

Adds an "Ecosystem / Adopters" section to track projects producing or consuming OKF
bundles, seeded with CATAAM. CATAAM's OKF Context Engine serialises an org's compliance
graph (requirements, controls, assets, tests, findings) into signed OKF bundles delivered
to a customer Git repo or as downloadable exports, consumed by AI agents via MCP and by
auditors.

Happy to move this to a dedicated ADOPTERS.md if maintainers prefer.

What I committed

README.md — a new ## Ecosystem / Adopters section (placed after Getting Started, before Contributing) with the CATAAM entry:

▎ CATAAM (https://cataam.com) — GRC and security platform. Its OKF Context Engine compiles an organization's compliance graph … into signed OKF bundles that AI agents (via MCP) and auditors consume…

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[avinashanand]

@googlebot I signed it!