Proposal: add Community Implementations section to README or SPEC

The [OKF blog post](https://cloud.google.com/blog/products/data-analytics/how-the-open-knowledge-format-can-improve-data-sharing) explicitly calls for the community to:

> - Write a producer for your source system
> - Write a consumer (viewer, search index, reasoning agent)
> - Contribute issues, PRs, or extensions

As people build these, there's no canonical place to discover them. I'd propose adding a **"Community Implementations"** section to either the README or SPEC.md.

**Why it helps:**
1. Gives new OKF adopters a starting point (they can study real implementations)
2. Creates a virtuous cycle — more listed tools → more adoption → more tools
3. Keeps the spec itself clean (no vendor bias) while the README is the natural place for ecosystem links
4. Directly fulfills the blog post's call to action

**Things to consider:**
- Should there be a bar for listing (e.g., "must claim OKF conformance")?
- Should tools be sorted by first-contribution date to avoid politicization?

Happy to PR this with initial entries if the direction is agreeable.