README doesn't mention GitHub Discussions now that it's enabled

#31 enabled Discussions with the default category set (General, Ideas, Q&A, Show and tell, Polls, Announcements), and `.github/ISSUE_TEMPLATE/config.yml` now links to the Q&A category. But README.md itself never mentions Discussions exists, so a reader who lands there first (which is far more likely than the issue-creation flow) has no way to find it. Add a short mention, most naturally next to the existing "Report a broken link" / "Changelog" links in the header blockquote, pointing at Discussions and specifically calling out Show and tell as where students who write their own skill should post it (per #31's rationale: that category is meant to be a contribution funnel, not just a chat room). **Scope:** tiny, roughly 10 minutes.