add dark-mode image

Author: samizdatco

README.md

@@ -1,4 +1,7 @@
-![Skia Canvas](/test/assets/readme-header@2x.png)
+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="test/assets/readme-header-dark@2x.png">
+  <img alt="Skia Canvas" src="test/assets/readme-header@2x.png">
+</picture>
 
 Skia Canvas is a browser-less implementation of the HTML Canvas drawing API for Node.js. It is based on Google’s [Skia](https://skia.org) graphics engine and as a result produces very similar results to Chrome’s `<canvas>` element. The library is well suited for use on desktop machines where you can render hardware-accelerated graphics to a window and on the server where it can output a variety of image formats.