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

@@ -1,8 +1,11 @@
 # Batomorph
 
-This is a simple raytracer, written for POSIX systems in C23 with zero
+Batomorph is a simple ray tracer, written for POSIX systems in C23
-runtime dependencies (outside of the standard library).  To build with
+with no runtime dependencies (outside of the standard library).
-CMake:
+
+![Demo rendering](demo.png)
+
+To build with CMake:
 
 ```sh
 cmake -B build