docs: add nix command to installation

thenbe · kojix2 · commit ef6794e32f39 · 2024-06-25T13:58:23.000+09:00
diff --git a/README.md b/README.md
@@ -18,6 +18,10 @@
 brew install youplot
 ```
 
+```
+nix shell nixpkgs#youplot
+```
+
 ```
 gem install youplot
 ```
