Add logo

Signed-off-by: Tim Paine <3105306+timkpaine@users.noreply.github.com>

Author: timkpaine

README.md

@@ -1,4 +1,9 @@
-# nepenthe
+<a href="https://github.com/point72/nepenthe">
+  <picture>
+    <source media="(prefers-color-scheme: dark)" srcset="https://github.com/point72/nepenthe/raw/main/docs/img/logo-dark.png?raw=true">
+    <img alt="nepenthe logo, overlapping blue speech bubbles" src="https://github.com/point72/nepenthe/raw/main/docs/img/logo-light.png?raw=true" width="400">
+  </picture>
+</a>
 
 Forget your environment sorrows