Update 'README.md'

README.md

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 # raytracer
 
-Based on code from the [Ray Tracing in One Weekend](https://raytracing.github.io/) book series.
+An implementation of raytracing in modern C++, based on code from the [Ray Tracing in One Weekend](https://raytracing.github.io/) book series. This implementation uses multi-threading, ranges, and [imgui](https://github.com/ocornut/imgui) to provide an interactive rendering experience.
 
+UI controls include:
+
+* Field of view
+* CPU thread/worker count
+* XYZ camera position
+* IJK camera "look at" position
+* Samples count to control render quality
+* Global shade control for "day" or "night" rendering
+* "balls" window: Add or remove spheres and set each sphere's material, size, and position
+
+The UI also does low-quality "live" rendering as settings are changed; click "recalculate" to render in high-quality. The visible render can be exported as a PNG image to the current directory.
+
+![](screenshot.png)
+
+## Build instructions
+
+1. Download imgui submodule via `git submodule`.
+2. Have GCC, GNU Make, and SDL2 installed.
+3. Run 'make'.
+
+The final program binary is called `main`.