Specified logo source for dark mode

It is now possible to specify different sources for images based on the user's theme preference. The black "Swift" text was a little unreadable on grey background, so I've adjusted it to display the white logo in dark mode.

Author: evrifaessa

README.md

@@ -1,4 +1,7 @@
-<img src="https://swift.org/assets/images/swift.svg" alt="Swift logo" height="70" >
+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="https://www.swift.org/assets/images/swift~dark.svg">
+  <img src="https://swift.org/assets/images/swift.svg" alt="Swift logo" height="70" >
+</picture>
 
 # Swift Programming Language