add readme for imgui_integration example

examples/imgui_integration/README.md

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+# ggdt - ImGui Integration Demo
+
+Borrows a lot of elements from the "slimed" demo, available in the `/examples` folder that this same demo is located
+within, to run a _somewhat_ simpler world and entity system from that demo while showing some bits of entity state
+in realtime using an ImGui-based display of this state information.
+
+The primary point of this demo is to show usage of the ggdt ImGui integration in a bigger application that somewhat
+resembles a "real" project (that is, using the application state management subsystem, the entity subsystem, etc) to
+show how this integration can be done.
+
+The main points of interest in this demo are found in the `main.rs` module.
+
+Simply do `cargo run` from this directory to try it out.
+
+![](screenshot.png)