Adding instruction for cloning repo with submodule.

Author: yuhui-zheng

Diff for: README.md

@@ -6,4 +6,23 @@ With FreeRTOS+POSIX, an existing POSIX threading compliant application can be po
 
 For more on FreeRTOS+POSIX, please visit [freertos.org](https://www.freertos.org/FreeRTOS-Plus/FreeRTOS_Plus_POSIX/index.html)
 
-The demo project is located under ```FreeRTOS-Plus\Demo\FreeRTOS_Plus_POSIX_with_actor_Windows_Simulator```. 
+The demo project is located under ```FreeRTOS-Plus\Demo\FreeRTOS_Plus_POSIX_with_actor_Windows_Simulator```. 
+
+## Cloning
+This repo uses [Git Submodules](https://git-scm.com/book/en/v2/Git-Tools-Submodules) to bring in dependent components.
+
+Note: If you download the ZIP file provided by GitHub UI, you will not get the contents of the submodules. (The ZIP file is also not a valid git repository)
+
+To clone using HTTPS:
+```
+git clone https://github.com/FreeRTOS/Lab-Project-FreeRTOS-POSIX.git --recurse-submodules
+```
+Using SSH:
+```
+git clone [email protected]:FreeRTOS/Lab-Project-FreeRTOS-POSIX.git --recurse-submodules
+```
+
+If you have downloaded the repo without using the `--recurse-submodules` argument, you need to run:
+```
+git submodule update --init --recursive
+```