docs: point runners at install script, document self-updates

Runtime box now leads with the tarball install_linux.sh (which installs the webkit runtime for supported distros) and keeps manual per-distro commands as fallback. Add an Updates section: desktop Linux releases self-update in-app (signed, verified), binary-only; icons and .desktop still come from the tarball.

README.md

@@ -11,6 +11,8 @@ To run as is, please feel free to download a binary from the releases page.
 > **To run:** install the WebKitGTK 6 runtime first — it is not preinstalled
 > on most distros, and the app will not start without it
 > (`error while loading shared libraries: libwebkitgtk-6.0.so.4`).
+> The release tarball's `install_linux.sh` installs it automatically
+> (Arch, Debian/Ubuntu, Fedora, Void, OpenMandriva). Manual fallback —
 > Arch: `sudo pacman -S webkitgtk-6.0` ·
 > Debian/Ubuntu: `sudo apt install libwebkitgtk-6.0-4`.
 
@@ -58,3 +60,10 @@ The `wailsv3` branch moves AniTrack from Wails `v2` → `v3` and from `99designs
 ## Building
 
 To build a redistributable, production mode package, use `make build`. The binary lands at `build/bin/AniTrack`.
+
+## Updates
+
+Desktop Linux releases self-update: the app checks for a newer release at
+startup and offers it in-app (signed, verified before install). Only the
+binary updates itself — icons and the `.desktop` file still come from the
+release tarball. Details in `docs/V3_MIGRATION.md: Self-updates`.