Update doc

src/hello_imgui/hello_imgui_api.md

@@ -132,6 +132,8 @@ int main(){
   Select the wanted backend type between `Sdl`, `Glfw` and `Qt`. Only useful when multiple backend are compiled
   and available.
 * `fpsIdling`: _FpsIdling_. Idling parameters (set fpsIdling.enableIdling to false to disable Idling)
+* `useImGuiTestEngine`: _bool, default=false_.
+   Set this to true if you intend to use imgui_test_engine (please read note below)
 * `iniFilename`: _string, default = ""_
   Sets the ini filename under which imgui will save its params. Path is relative to the current app working dir.
   If empty, then the ini file name will be derived from appWindowParams.windowTitle (if both are empty, the ini filename will be imgui.ini).
@@ -145,6 +147,13 @@ int main(){
 * `emscripten_fps`: _int, default = 0_.
   Set the application refresh rate (only used on emscripten: 0 stands for "let the app or the browser decide")
 
+Notes about the use of [Dear ImGui Test & Automation Engine](https://github.com/ocornut/imgui_test_engine):
+* HelloImGui must be compiled with the option HELLOIMGUI_WITH_TEST_ENGINE (-DHELLOIMGUI_WITH_TEST_ENGINE=ON)
+* See demo in src/hello_imgui_demos/hello_imgui_demo_test_engine.
+* imgui_test_engine is subject to a [specific license](https://github.com/ocornut/imgui_test_engine/blob/main/imgui_test_engine/LICENSE.txt)
+  TL;DR: free for individuals, educational, open-source and small businesses uses. Paid for larger businesses.
+
+
 #### Fps Idling
 
 **FpsIdling** is a struct that contains Fps Idling parameters