Remove mention of Playground being a publicly editable wiki from CONTRIBUTING.md

Arduino decided to make the Arduino Playground read only, so it is no longer possible for people to edit it directly. They will now need to report issues or suggestions for improvement to the arduino/Arduino issue tracker so that someone with edit privileges can make the required changes.

Author: per1234

CONTRIBUTING.md

@@ -6,7 +6,6 @@ Thanks for your interest in contributing to this free open source project! Ardui
 | - Support request<br/>- Question<br/>- Problem with your Arduino<br/>- Discussion | Post on the [Arduino Forum](http://forum.arduino.cc) |
 | - Bug report<br/>- [Arduino website](https://www.arduino.cc/) issue or improvement<br/>- Feature request | Issue report (read the [issue guidelines](#issues)) |
 | - 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) |
@@ -19,7 +18,7 @@ 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) |
+| Arduino IDE, Arduino AVR Boards, arduino.cc (but not the Arduino 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) |