Add license info as MIT

Author: asukiaaa

README.md

@@ -86,6 +86,9 @@ If you want to use code for multiple platform including any board that does not
 #endif
 ```
 
+# License
+MIT
+
 # References
 - [HardwareSerial.h](https://github.com/arduino/Arduino/blob/master/hardware/arduino/avr/cores/arduino/HardwareSerial.h)
 - [SoftwareSerial.h](https://github.com/arduino/Arduino/blob/master/hardware/arduino/avr/libraries/SoftwareSerial/src/SoftwareSerial.h)