[Keyboard] Add Splitty keyboard (#11613)

keyboards/splitty/keymaps/default/readme.md

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+# The default keymap for Splitty
+
+Simple default keymap.
+
+Hold the space key to access the navigation layer. Hold the backspace key for
+left control.
+
+There is a Ctrl-W shortcut which is usefull to erase previous word and for
+window management in vim.