README: white logo for dark mode

white logo was approved in https://github.com/NixOS/nixos-artwork/pull/80

README.md

@@ -1,5 +1,10 @@
 <p align="center">
-  <a href="https://nixos.org/nixos"><img src="https://nixos.org/logo/nixos-hires.png" width="500px" alt="NixOS logo" /></a>
+  <a href="https://nixos.org#gh-light-mode-only">
+    <img src="https://raw.githubusercontent.com/NixOS/nixos-homepage/master/logo/nixos-hires.png" width="500px" alt="NixOS logo"/>
+  </a>
+  <a href="https://nixos.org#gh-dark-mode-only">
+    <img src="https://raw.githubusercontent.com/NixOS/nixos-artwork/master/logo/nixos-white.png" width="500px" alt="NixOS logo"/>
+  </a>
 </p>
 
 <p align="center">