Merge pull request arduino#165 from arduino/aentinger-patch-2

Fix: Providing documentation for various cores how to manually integrate ArduinoCore-API.

Author: facchinm

README.md

@@ -77,6 +77,11 @@ In order to compile a core which is implementing ArduinoCore-API you'll need to
 tar --exclude='*.git*' -cjhvf $yourcore-$version.tar.bz2 $yourcore/
 ```
 
+Documentation for how to integrate with a Arduino core (which is necessary if you do not download the Arduino core via the Boards Manager) can be found here:
+* [ArduinoCore-megaavr](https://github.com/arduino/ArduinoCore-megaavr#developing)
+* [ArduinoCore-mbed](https://github.com/arduino/ArduinoCore-mbed#clone-the-repository-in-sketchbookhardwarearduino-git)
+* [ArduinoCore-samd](https://github.com/arduino/ArduinoCore-samd/#developing)
+
 ## Donations
 
 This open source code is maintained by Arduino with the help of the community. We invest a considerable amount of time in testing code, optimizing it and introducing new features. Please consider [donating](https://www.arduino.cc/en/donate/) or [sponsoring](https://github.com/sponsors/arduino) to support our work, as well as [buying original Arduino boards](https://store.arduino.cc) which is the best way to make sure our effort can continue in the long term.