Add Language Reference repositories to CONTRIBUTING.md

The reference-en repository is now being used to generate the Language Reference pages. The Language Reference translation projects also seem to be progressing.

Author: per1234

CONTRIBUTING.md

@@ -8,6 +8,7 @@ Thanks for your interest in contributing to this free open source project! Ardui
 | - Bug fix<br/>- Enhancement | Pull Request (read the [pull request guidelines](#pull-requests)) |
 | [Arduino Playground](http://playground.arduino.cc/) | This is a publicly editable wiki. Feel free to make edits or create a post on the [forum](http://forum.arduino.cc/index.php?board=24.0) to discuss changes. |
 | Translations for the Arduino IDE | [transifex](https://www.transifex.com/mbanzi/arduino-ide-15/) |
+| Translations for the [Language Reference](https://www.arduino.cc/reference) | [Reference repositories](https://github.com/arduino?q=reference-) |
 | Monetary | - [Donate](https://www.arduino.cc/en/Main/Contribute)<br/>- [Buy official products](https://store.arduino.cc) |
 
 
@@ -19,6 +20,7 @@ Thanks for your interest in contributing to this free open source project! Ardui
 | Issue topic | Report at |
 |-|-|
 | Arduino IDE, Arduino AVR Boards, arduino.cc (but not the Arduino Playground or forum), Library Manager additions | [arduino/Arduino](https://github.com/arduino/Arduino/issues) |
+| [Language Reference](https://www.arduino.cc/reference) | [Reference repositories](https://github.com/arduino?q=reference-) |
 | Arduino Forum | [arduino/forum-issues](https://github.com/arduino/forum-issues/issues) |
 | Arduino libraries | [arduino-libraries](https://github.com/arduino-libraries) |
 | arduino-builder | [arduino/arduino-builder](https://github.com/arduino/arduino-builder/issues) |