Adjust instruction for setting custom $ZDOTDIR

Adjust instruction for setting custom `$ZDOTDIR`.

This addresses 2 potential issues:
- Check for the presence of `$XDG_CONFIG_HOME/zsh` before assigning it to `$ZDOTDIR` instead of blindly assigning
- Avoid recursion in zsh variable assignment stack that might occasionally result in messages like:
```
zsh: job table full or recursion limit exceeded
```

README.md

@@ -41,7 +41,7 @@ version is **4.3.11**.
 
         ```sh
         export XDG_CONFIG_HOME="${XDG_CONFIG_HOME:=$HOME/.config}"
-        export ZDOTDIR="${ZDOTDIR:=$XDG_CONFIG_HOME/zsh}"
+        [[ -d $XDG_CONFIG_HOME/zsh ]] && export ZDOTDIR="$XDG_CONFIG_HOME/zsh"
         source "$ZDOTDIR/.zshenv"
         ```