Replacing readme.txt with README.md.

README.md

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+## Getting started
+This repository contains FreeRTOS kernel source/header files and kernel ports only. This repository is referenced as a submodule in [FreeRTOS/FreeRTOS](https://github.com/FreeRTOS/FreeRTOS) repository, which contains pre-configured demo application projects under ```FreeRTOS/Demo``` directory. 
+
+The easiest way to use FreeRTOS is to start with one of the pre-configured demo application projects.  That way you will have the correct FreeRTOS source files included, and the correct include paths configured.  Once a demo application is building and executing you can remove the demo application files, and start to add in your own application source files.  See the [FreeRTOS Kernel Quick Start Guide](https://www.freertos.org/FreeRTOS-quick-start-guide.html) for detailed instructions and other useful links.
+
+Additionally, for FreeRTOS kernel feature information refer to the [Developer Documentation](https://www.freertos.org/features.html), and [API Reference](https://www.freertos.org/a00106.html).
+
+### Getting help
+If you have any questions or need assistance troubleshooting your FreeRTOS project, we have an active community that can help on the [FreeRTOS Community Support Forum](https://forums.freertos.org).
+
+## Cloning this repository
+
+To clone using HTTPS:
+```
+git clone https://github.com/FreeRTOS/FreeRTOS-Kernel.git
+```
+Using SSH:
+```
+git clone [email protected]:FreeRTOS/FreeRTOS-Kernel.git
+```
+
+## Repository structure
+- The root of this repository contains the three files that are common to 
+every port - list.c, queue.c and tasks.c.  The kernel is contained within these 
+three files.  croutine.c implements the optional co-routine functionality - which
+is normally only used on very memory limited systems.
+
+- The ```./portable``` directory contains the files that are specific to a particular microcontroller and/or compiler. 
+See the readme file in the ```./portable``` directory for more information.
+
+- The ```./include``` directory contains the real time kernel header files.