Add note about updating submodule

Author: Jamie Smith

README.md

@@ -4,9 +4,10 @@ This project implements a basic hello world using Mbed OS Community Edition.  Us
 ## How to set up this project:
 
 1. Clone it to your machine.  Don't forget to use `--recursive` to clone the submodules: `git clone --recursive https://github.com/mbed-ce/mbed-ce-hello-world.git`
-2. Set up the GNU ARM toolchain (and other programs) on your machine using [the toolchain setup guide](https://github.com/mbed-ce/mbed-os/wiki/Toolchain-Setup-Guide).
-3. Set up the CMake project for editing.  We have three ways to do this:
+2. You may want to update the mbed-os submodule to the latest version, with `cd mbed-os && git pull origin master`
+3. Set up the GNU ARM toolchain (and other programs) on your machine using [the toolchain setup guide](https://github.com/mbed-ce/mbed-os/wiki/Toolchain-Setup-Guide).
+4. Set up the CMake project for editing.  We have three ways to do this:
     - On the [command line](https://github.com/mbed-ce/mbed-os/wiki/Project-Setup:-Command-Line)
     - Using the [CLion IDE](https://github.com/mbed-ce/mbed-os/wiki/Project-Setup:-CLion)
     - Using the [VS Code IDE](https://github.com/mbed-ce/mbed-os/wiki/Project-Setup:-VS-Code)
-4. Build the `flash-HelloWorld` target to upload the code to a connected device.
+5. Build the `flash-HelloWorld` target to upload the code to a connected device.