Add Windows themes folder

Co-authored-by: digidoor <37601466+digidoor@users.noreply.github.com>

book/src/themes.md

@@ -9,7 +9,7 @@ To use a theme add `theme = "<name>"` to the top of your [`config.toml`](./confi
 To create a theme file:
 
 1. Create a 'themes' folder in your user configuration folder (e.g.
-   `~/.config/helix/themes`).
+   `~/.config/helix/themes`, or `%AppData%\helix\themes` on Windows).
 2. Create a file with the name of your theme as the file name (e.g.
    `mytheme.toml`) and place it in your `themes` folder.