1. Update README

Author: onexs-xsi

README.md

@@ -14,6 +14,28 @@ M5IOE1 is a versatile I2C-based I/O expansion library designed for M5Stack devic
 
 - N/A
 
+## Notes
+
+### Include Order (ESP-IDF)
+
+If used alongside `M5Unified`, include it **before** `M5IOE1`:
+
+```cpp
+#include <M5Unified.h>   // ✓ must come first
+#include <M5IOE1.h>
+```
+
+```cpp
+#include <M5IOE1.h>       // ✗ causes i2c_config_t conflict
+#include <M5Unified.h>
+```
+
+> **Why:** On ESP-IDF ≥ 5.3.0 without `CONFIG_I2C_BUS_BACKWARD_CONFIG`, `i2c_bus.h` defines
+> its own `i2c_config_t`. Including it before `M5Unified` (which pulls in `driver/i2c.h`)
+> creates a conflicting declaration error. Reversing the order avoids this.
+>
+> Alternatively, enable `CONFIG_I2C_BUS_BACKWARD_CONFIG` in menuconfig to remove the restriction.
+
 ## License
 
 - [M5IOE1 - MIT](LICENSE)