Add a link to the image sources repo

We can add a link to https://github.com/w3c/jlreq-image-sources in the [readme](https://github.com/w3c/jlreq?tab=readme-ov-file#links-for-editors%E3%83%89%E3%82%AD%E3%83%A5%E3%83%A1%E3%83%B3%E3%83%88%E7%B7%A8%E9%9B%86%E3%81%AE%E3%81%9F%E3%82%81%E3%81%AE%E3%83%AA%E3%83%B3%E3%82%AF) and/or [EDITING.md](https://github.com/w3c/jlreq/blob/gh-pages/EDITING.md).