chore: add checkout instructions

Author: rfletchr

README.md

@@ -2,6 +2,18 @@
 This project provides a simple jumping off point for writing ImGui apps with C++.
 Window management is provided by Raylib, which compiles out of the box on most systems.
 
+## Usage
+Checkout the repsitory, making sure to properly initialise the submodules.
+```
+https://github.com/rfletchr/cpp_imgui_starter_app.git my_app
+cd my_app
+git submodule init
+git submodule update --recursive
+cmake --preset debug
+cmake --build --preset debug
+```
+
+
 ## Missing Dependencies
 In cases where configuration fails due to missing dependencies Raylib provides a guide [here](https://github.com/raysan5/raylib/wiki/raylib-dependencies#platform-specific-dependency-notes).