Write README

.gitattributes

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+demo.png filter=lfs diff=lfs merge=lfs -text

.gitignore

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 build/
 *.ff
 *.png
+!demo.png

README.md

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+# Batomorph
+
+Batomorph is a simple ray tracer, written in C23 for POSIX systems.
+
+![Demo rendering](demo.png)
+
+To build with CMake (there are no dependencies):
+
+```sh
+cmake -B build
+cmake --build build
+```
+
+You may want to adjust the `NTHREADS` definition in `src/render.c` to
+match your system.
+
+The demo programs (`build/demo` and `build/rand`) will write the
+rendered image in sRGB
+[Farbfeld](https://tools.suckless.org/farbfeld/) to `stdout`:
+
+```sh
+build/demo > out.ff           # Produce Farbfeld file
+build/demo | ff2png > out.png # Produce PNG file
+build/demo | feh              # View directly
+```
+
+(Note that `ff2png` and `feh` are third-party programs)