[doc] add a simple way to make a coord_mapping

docs/porting_to_kmk.md

@@ -63,6 +63,48 @@ coord_mapping = [
 ]
 ```
 
+Note: Not all numbers are necessarily used ! The keyboard assumes
+`number of line * number of rows` keys. Some of the possible keys might not be
+used. For example a keyboard with 60 keys might have 8 rows, 8 cols, allowing
+64 total combinations -- hence 64 keys. 4 numbers will then not be used for keys
+in the `coord_mapping` (might be anyone of them depending of the wiring).
+
+### Find your coord mapping
+The following code will help you setup your `coord_mapping` by having every key
+send its corresponding number. Use it after your pins and module definition
+to define both `keyboard.coord_mapping` and `keyboard.keymap`.
+
+```python
+from kmk.handlers.sequences import simple_key_sequence
+from kmk.keys import KC
+
+# *2 for split keyboards, which will typically manage twice the number of keys
+# of one side. Having this N too large will have no impact (maybe slower boot..)
+N = len(keyboard.col_pins) * len(keyboard.row_pins) * 2
+
+keyboard.coord_mapping = list(range(N))
+
+layer = []
+
+for i in range(N):
+    c, r = divmod(i, 100)
+    d, u = divmod(r, 10)
+    layer.append(
+        simple_key_sequence(
+            (
+                getattr(KC, 'N' + str(c)),
+                getattr(KC, 'N' + str(d)),
+                getattr(KC, 'N' + str(u)),
+                KC.SPC,
+            )
+        )
+    )
+keyboard.keymap = [layer]
+
+if __name__ == '__main__':
+    keyboard.go()
+```
+
 ## Keymaps
 Keymaps are organized as a list of lists. Keycodes are added for every key on 
 each layer. See [keycodes](keycodes.md) for more details on what keycodes are