docs: add example avante with nixvim options (#1561)

README.md

@@ -230,6 +230,19 @@ programs.neovim = {
 
 </details>
 
+<details>
+
+  <summary><a href="https://nix-community.github.io/nixvim/plugins/avante/index.html">Nixvim</a></summary>
+
+```nix
+  plugins.avante.enable = true;
+  plugins.avante.settings = {
+    # setup options here
+  };
+```
+
+</details>
+
 <details>
 
   <summary>Lua</summary>