Document self-updates

Co-Authored-By: Claude Opus 5.5 <[email protected]>

README.md

@@ -135,6 +135,10 @@ Release builds send anonymous usage statistics (Aptabase) and crash reports (Bug
 servers in the repository secrets `APTABASE_HOST`, `APTABASE_APP_KEY` and `BUGSINK_DSN`. Builds without them,
 including local ones, send nothing.
 
+Installs update themselves: when T5DE starts, it looks for a newer build on its channel (releases for releases,
+the nightly for nightlies) and offers it in the client's update dialog. Updating downloads the new setup, which
+reapplies the install's patches and settings. Local builds don't update.
+
 `.gitea/workflows/imvu-update.yml` runs `t5de check-update` daily and opens an issue when imvu.com has a newer IMVU
 Classic release than the one pinned in `t5de.toml`.