discourse-release: announce a given tag, for releases made by the job's own token

Gitea starts no workflow for events caused by the Actions bot, so a release
created with GITHUB_TOKEN never fires 'on: release'. With 'tag:' the action
fetches that release itself; drafts are skipped.

README.md

@@ -38,6 +38,19 @@ jobs:
           discord-webhook: ${{ secrets.DISCORD_RELEASE_WEBHOOK }}   # optional
 ```
 
+A workflow that creates its release with the job's own token (`secrets.GITHUB_TOKEN`)
+must announce it itself, as a last step: Gitea starts no workflow for events
+the Actions bot causes, so `on: release` never fires for such a release.
+
+```yaml
+      - uses: coffey-labs/actions/discourse-release@<sha>
+        with:
+          api-key: ${{ secrets.DISCOURSE_RELEASE_KEY }}
+          tag: ${{ github.ref_name }}
+```
+
+Both at once is fine: a repo+tag is announced once, whichever runs first.
+
 - The repo must be listed in `discourse-release/release-map.json`, the one
   place that maps repos to forum categories.
 - `DISCOURSE_RELEASE_KEY` is an org-level secret (coffey-labs and inbuxa)