Add sample image

README.md

 # Raytracing with Rust
 
 A crude raytracer implemented by following https://raytracing.github.io/books/RayTracingInOneWeekend.html.
+
+![A sample image](images/sample.png)

src/main.rs

@@ -40,12 +40,12 @@ use message::Message;
 fn main() {
     // Image
     let aspect_ratio = 16.0 / 9.0;
-    let image_width = 400;
+    let image_width = 1920;
     let image_height = (image_width as f64 / aspect_ratio) as usize;
     let samples_per_pixel = 100;
     let max_depth = 50;
     let chunk_size = 20000usize;
-    let threads = 4;
+    let threads = 12;
 
     // World
     let mut world = HittableList::new();