Add a README explaining what this repository is

README.md

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+# Arduino Core for SAM3X CPU
+
+This repository contains the source code and configuration files of the Arduino Core for Atmel's SAM3X processor (used on the [Arduino Due](https://www.arduino.cc/en/Main/ArduinoBoardDue) board).
+
+## Installation on Arduino IDE
+
+This core is available as a package in the Arduino IDE cores manager.
+Just open the "Boards Manager" and install the package called:
+
+ * **Arduino SAM Boards (32-bit ARM Cortex-M3)**
+
+## Support
+
+There is a dedicated section of the Arduino Forum for general discussion and project assistance:
+
+http://forum.arduino.cc/index.php?board=87.0
+
+## Bugs or Issues
+
+If you find a bug you can submit an issue here on github:
+
+https://github.com/arduino/ArduinoCore-sam/issues
+
+Before posting a new issue, please check if the same problem has been already reported by someone else to avoid duplicates.
+
+## Contributions
+
+Contributions are always welcome.
+The preferred way to receive code contribution is by submitting a Pull Request on github.