docs: note Linux screen-share audio limitation + PipeWire workaround

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -22,6 +22,16 @@ Latest build for your platform on the **[Releases page](https://codeberg.org/Enk
 - **Linux** can't self-update (owned by the package manager / flatpak sandbox), so Blap **notifies**
   you and you update with one command — see **[HOW-TO-UPDATE.md](./HOW-TO-UPDATE.md)**.
 
+## Screen sharing audio
+
+- **Windows / macOS** — system audio is shared with your screen automatically.
+- **Linux** — Electron can't capture system audio for screen shares (a longstanding
+  Electron/Chromium limitation, not specific to Blap). Workaround on PipeWire (Debian, Bazzite,
+  etc.): in the call, open the **mic device dropdown** (the ⌄ next to the mic button) and pick
+  **"Monitor of \<your output device\>"** as your input — your system audio then rides in on the
+  mic channel. Note this replaces your microphone, so you share audio *or* talk (use `qpwgraph` /
+  `pavucontrol` to combine both if you need it).
+
 ## Building
 
 Requires Node ≥ 22 and pnpm. From the repo root: