Added a link to the community supported demos repo in the demos readme.txt (#987)

* Added a link to the community supported demos repo in the demos readme.txt

* Update FreeRTOS/Demo/readme.txt

Co-authored-by: Aniruddha Kanhere <60444055+AniruddhaKanhere@users.noreply.github.com>

* Update and rename readme.txt to readme.md

---------

Co-authored-by: Aniruddha Kanhere <60444055+AniruddhaKanhere@users.noreply.github.com>

FreeRTOS/Demo/readme.md

@@ -1,3 +1,4 @@
+# Documentation
 Links to a documentation page for each demo are provided on the following
 URL: http://www.freertos.org/a00090.html
 
@@ -11,9 +12,15 @@ every port.
 + All the other directories contain a project or makefile for the demo
 application targeted at a particular microcontroller.  
 
-
 For example, if you are interested in the ATMega323 demo application for
 the WinAVR tools then the AVR_ATMega323_WinAVR directory contains the 
 relevant makefile.  The makefile includes files from the Demo/ATMega323 
 and the Demo/Common directories.  If this is the only port you are 
 interested in then all the other directories can be ignored.
+
+# Community Supported Demos
+Not all demos are maintained by the FreeRTOS team in the FreeRTOS/FreeRTOS repo.
+Community Supported Demos can be found here:
+https://github.com/FreeRTOS/FreeRTOS-Community-Supported-Demos
+
+If you would like to contribute by adding a demo, please see this guide to get to know the prerequisites and to get information on how to structure the directories: https://github.com/FreeRTOS/FreeRTOS/blob/main/FreeRTOS/Demo/ThirdParty/Template/README.md