Add demo image to README

.gitattributes

@@ -0,0 +1 @@
+demo.png filter=lfs diff=lfs merge=lfs -text

.gitignore

@@ -1,3 +1,4 @@
 build/
 *.ff
 *.png
+!demo.png

README.md

@@ -0,0 +1,29 @@
+# Batomorph
+
+Batomorph is a simple ray tracer, written for POSIX systems in C23
+with no runtime dependencies (outside of the standard library).
+
+![Demo rendering](demo.png)
+
+To build with CMake:
+
+```sh
+cmake -B build
+cmake --build build
+```
+
+You may want to adjust the `NTHREADS` definition in `src/render.c` to
+match your system for best performance.
+
+When ran, the demo programs (`build/demo` and `build/rand`) will write
+the rendered image in sRGB
+[Farbfeld](https://tools.suckless.org/farbfeld/) to standard out.  For
+example:
+
+```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)