Added fish way to add PATH to README.md

Author: sato-s

README.md

@@ -58,6 +58,10 @@ zsh:
 ```console
 $ echo 'export PATH="$HOME/.tfenv/bin:$PATH"' >> ~/.zprofile
 ```
+fish:
+```console
+$  echo 'set -x PATH $HOME/.tfenv/bin $PATH' >> ~/.config/fish/config.fish
+```
 
 For WSL users:
 ```bash