Document XDG_CONFIG_HOME support on mac

fix getsops/sops#1896

Converted from getsops/sops@ca4fef7

Co-authored-by: Frederic R. <zougi.fr@gmail.com>
Signed-off-by: Frederic R. <zougi.fr@gmail.com>
Signed-off-by: Felix Fontein <felix@fontein.de>

Author: felixfontein

content/en/docs/_index.md

@@ -240,7 +240,8 @@ user configuration directory.
 
 * **macOS**
 
-  - Looks for `keys.txt` in `$HOME/Library/Application Support/sops/age/keys.txt`.
+  - Looks for `keys.txt` in `$XDG_CONFIG_HOME/sops/age/keys.txt`;
+  - Falls back to `$HOME/Library/Application Support/sops/age/keys.txt`.
 
 * **Windows**
   - Looks for `keys.txt` in `%AppData%\\sops\\age\\keys.txt`.