Update msiu's xd75 keymap README to be accurate (#6443)

keyboards/xd75/keymaps/msiu/readme.md

@@ -1 +1,3 @@
-# The default keymap for xd75
+# msiu's keymap for xd75
+
+QWERTY keymap for xd75 with centered numpad and standard 40% layers for easy switching between xd75 and contra/planck.