README: document second-machine (Syncthing) setup

Add a "Running on a second machine" section: clone + install, point the
per-machine ~/.config/lintunes/config.json at the synced folder via --save-config
(no re-import needed since paths are stored relative to the data dir), and run
packaging/install-desktop.sh to install + pin the launcher. Note the desktop
launcher follows the saved config, and add smart playlists + multi-machine sync
to the feature list.

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

README.md

@@ -8,7 +8,9 @@ software for bugs and vulns?).
 
 ## Features
 
-- can import an  iTunes 12 (untested on other versions) library
+- can import an  iTunes 12 (untested on other versions) library, including
+  smart playlists (auto-updating, editable, marked with a ❧)
+- runs on several machines off one Syncthing-synced library (see below)
 
 
 
@@ -66,6 +68,40 @@ Fedora/Debian note: you need the FFmpeg codecs for Qt Multimedia
 LinTunes looks best with **Century Gothic** installed (`~/.local/share/fonts/`);
 if it's missing, you'll be asked to pick a font on first run.
 
+The desktop launcher runs `lintunes` with no `--data-dir`, so it always uses
+whatever is saved in `~/.config/lintunes/config.json`. Point that config wherever
+you want and the dock icon follows.
+
+## Running on a second machine (Syncthing)
+
+Keep the data dir (which holds `library.json`, the playlists, and
+`preferences.json`) **inside** your Syncthing-shared music folder, so the music
+and the library travel together. Track paths are stored *relative to the data
+dir*, so the library resolves correctly no matter where each machine mounts the
+shared folder — you never edit anything inside the library to move it.
+
+`~/.config/lintunes/config.json` is **per-machine** (it is *not* synced), so on a
+new machine you just tell LinTunes where the synced folder landed:
+
+```sh
+# 1. get the code
+git clone ssh://git@git.autonomic.zone:2222/trav/lintunes.git
+cd lintunes && pip install -e .
+
+# 2. let Syncthing finish replicating the music folder, then point the config at
+#    THIS machine's paths and launch — no re-import, the data is already synced:
+lintunes --data-dir  "/path/to/synced/music/lintunes" \
+         --music-root "/path/to/synced/music/iTunes Media" \
+         --save-config
+
+# 3. install the launcher + icon, then pin it (see "App icon" below)
+bash packaging/install-desktop.sh
+```
+
+(Step 2 just writes `data_dir` + `music_root` into `~/.config/lintunes/config.json`
+— you can also create that file by hand. After it's saved, every launch, including
+the dock icon, uses the synced library automatically.)
+
 ## Keys
 
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