docs: Add configuration option for Fish 3.2.0+ (#2449)

The original instructions were added 2 years ago in 0f2d659732. Since then, [Fish released](https://github.com/fish-shell/fish-shell/releases/tag/3.2.0) [a more declarative instruction for this operation](https://fishshell.com/docs/current/cmds/fish_add_path.html), which is now the recommended way of manipulating `PATH` in Fish.

README.md

@@ -324,14 +324,21 @@ See [Advanced configuration](#advanced-configuration) for details and more confi
 
   - For **Fish shell**:
 
-    Execute this interactively:
+    If you have Fish 3.2.0 or newer, execute this interactively:
+
+    ~~~ fish
+    set -Ux PYENV_ROOT $HOME/.pyenv
+    fish_add_path $PYENV_ROOT/bin
+    ~~~
+
+    Otherwise, execute the snippet below:
 
     ~~~ fish
     set -Ux PYENV_ROOT $HOME/.pyenv
     set -U fish_user_paths $PYENV_ROOT/bin $fish_user_paths
     ~~~
 
-    And add this to `~/.config/fish/config.fish`:
+    Now, add this to `~/.config/fish/config.fish`:
 
     ~~~ fish
     pyenv init - | source