Added screenshot for Cmake GUI

BUILDING.md

@@ -2,7 +2,7 @@
 
 ## On Windows
 1. Just open this project folder in Visual Studio 2019 (or newer), with 
-CMake Extension installed as part of "Desktop C++ Dev Workload"
+CMake plugin installed.
 2. From top toolbar, change mode to **Release**, and click **Build** > **Build All**.
 3. Done. That's it. Your .EXE will be in `{project_dir}/out/Release/bin`
 4. If the game fails to start, make sure `openal32.dll` is present right beside your executable. If not, copy it from SFML folder `/bin/openal32.dll`
@@ -19,3 +19,17 @@ cmake . . -DCMAKE_BUILD_TYPE=Release
 cmake --build . --config Release --target all 
 ```
 4. Your executable (.app bundle for MacOS) and binary for Linux will be found in `{project_dir}/build/bin`
+
+## Using CMake GUI (all destkop platforms)
+
+- Download CMake GUI installer for your OS from the official site. (supported on all desktop OS)
+- On MacOS, to add Cmake CLI to path, run the following command:
+  ```bash
+   sudo "/Applications/CMake.app/Contents/bin/cmake-gui" --install
+  ```
+- This is how the home screen of CMake GUI looks like:
+![cmake_screenshot](res/cmake_gui.png)
+
+- Set "Source Folder" to this project root. Then set "Build Folder" to a NEW _relative_ folder `/build` or `/out`. See image above. Then click "**Configure**", 
+- In the next screen, Choose **Unix Makefiles** on Linux & Mac. Choose **Visual Studio** on Windows. Then click OK. Click Configure once more, then **Generate**.
+- Finally, open build folder, then run `make all` in Terminal (Linux and macOS). On Windows, double-click the generated VS project `sln` file.