docs(README): note Tab/Shift+Tab cell navigation

Tab and Shift+Tab move focus between cells via the browser's native
tabindex traversal rather than registered Logseq commands, so they
can't be rebound from the Keymap UI. Document this alongside the
existing caret-navigation shortcuts.

Author: VictorVow

README.md

@@ -73,6 +73,8 @@ All shortcuts below are registered as Logseq commands, so you can rebind them fr
 | Move caret to cell left  | `Ctrl+Alt+Left`                    |
 | Move caret to cell right | `Ctrl+Alt+Right`                   |
 
+`Tab` and `Shift+Tab` also move focus to the next / previous cell. These use the browser's native focus traversal (each cell is a tab stop), so they aren't registered as Logseq commands and can't be rebound from the Keymap UI.
+
 ### Insert rows & columns
 
 | Action              | Default                |