Add a warning about the keymap.c getting overwritten.

lib/python/qmk/keymap.py

@@ -8,6 +8,11 @@ from .errors import NoSuchKeyboardError
 # The `keymap.c` template to use when a keyboard doesn't have its own
 DEFAULT_KEYMAP_C = """#include QMK_KEYBOARD_H
 
+/* THIS FILE IS AUTOMATICALLY GENERATED!
+ *
+ * Changes to this file will be overwritten. Modify keymap.json instead.
+ */
+
 const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
 __KEYMAP_GOES_HERE__
 };