[examples] add readme for screenshot example

apps/examples/screenshot_example/README.md

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+# Screenshot Example
+
+![screenshot](screenshot.jpg)
+
+A twisted take on "Hello World": reprojects an [Equirectangular
+Projection][wiki-equi] so that the North Pole lands where you click.
+
+You can save the reprojected map to a `.png` file by hitting the <kbd>S</kbd>
+key. Saving the current screen contents to an image is done using
+`le_screenshot`, for which this is the canonical example.
+
+Note how grid lines are drawn entirely in the shader.
+
+> [!TIP]
+> 
+> Take a look at `le_screenshot.h` where you find some more information on
+> screenshots, and also some hints for saving screenshots (or image sequences)
+> in higher bit-per-channel resolutions, and/or formats other than `.png`.
+
+## Techniques used: 
+
+* image loading using `le_resource_manager`
+* shader-based reprojection
+* fine lines drawn using a fragment shader (see also
+  [`exr_decode_example`](../examples/exr_decode_example/))
+* `le_debug_print_text` to print on-screen help message
+
+## Build instructions
+
+> [!IMPORTANT]
+> 
+> This example depends on one external image asset. To keep the Island code
+> repository slim, this asset is hosted outside of the repo.
+
+
+Download image assets via the bash script in the project root folder. 
+
+    ./download_assets.sh
+
+Configure build environment using CMake: 
+
+    mkdir build 
+    cd build
+    cmake -G Ninja ..
+
+Note that if you are using Qt Creator you may skip manually setting up the
+build environment, and simply open the project CMakeLists.txt using Qt Creator.
+
+Build using Ninja:
+
+    ninja
+
+Run application: 
+
+    ./Island-ScreenshotExample
+
+[wiki-equi]: https://en.wikipedia.org/wiki/Equirectangular_projection