docs(README): document radial's pointer-follows-focus warping

The radial menu warps the pointer to the centre of the newly active pane after
a split/close/focus-move slice (X11 QCursor::setPos; Wayland pointer-constraints,
as plan9port does), so the mouse follows keyboard/pane focus — acme-style.

README.md

@@ -212,6 +212,12 @@ through a list.
 - **Choose:** move toward a slice and release, or click it. A branch slice
   (marked ▸) opens a submenu; the centre hub goes back / cancels.
 - Slices can trigger any action by name, including `palette`/`switch` doors.
+- **Pointer follows focus.** When a slice changes the active pane (split, close,
+  or focus-move), the pointer is warped to the centre of the new pane so the
+  mouse stays where your attention is — the same cursor-follows-focus feel as
+  acme. On X11 this is a direct `QCursor::setPos`; on Wayland (which forbids
+  that) it uses the pointer-constraints protocol, matching how plan9port warps
+  the cursor there.
 - **Configure:** drop a `radials.json` at `~/.config/deft/radials.json` to
   replace the built-in tree; when it is absent, empty, or invalid, the built-in
   default is used. See [docs/radials.example.json](docs/radials.example.json)