Update README.md, add dark mode logo

README.md

@@ -1,4 +1,10 @@
-<p align="center"><img src="https://pixelfed.nyc3.cdn.digitaloceanspaces.com/logos/pixelfed-full-color.svg" width="300px"></p>
+<p align="center">
+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="https://pixelfed.nyc3.cdn.digitaloceanspaces.com/logos/pixelfed-full-color-dark.svg">
+  <source media="(prefers-color-scheme: light)" srcset="https://pixelfed.nyc3.cdn.digitaloceanspaces.com/logos/pixelfed-full-color.svg">
+  <img alt="Pixelfed logo" src="https://pixelfed.nyc3.cdn.digitaloceanspaces.com/logos/pixelfed-full-color.svg">
+</picture>
+</p>
 
 <p align="center">
 <a href="https://packagist.org/packages/pixelfed/pixelfed"><img src="https://poser.pugx.org/pixelfed/pixelfed/v/stable.svg" alt="Latest Stable Version"></a>