List ArduinoCore-renesas as core using ArduinoCore-API (arduino#215)

Author: aentinger

README.md

@@ -13,6 +13,7 @@ As of now, the following official cores are utilising ArduinoCore-API:
 * [megaavr](https://github.com/arduino/ArduinoCore-megaAVR)
 * [mbed](https://github.com/arduino/ArduinoCore-mbed)
 * [samd](https://github.com/arduino/ArduinoCore-samd)
+* [renesas](https://github.com/arduino/ArduinoCore-renesas)
 
 There's an ongoing effort to port the others, while maintainers of third-party cores are strongly invited to follow the same route in order to stay up-to-date with the new language features. For backwards compatibility, every revision of this repo will increase the `ARDUINO_API_VERSION` define.