specify default XDG folders

Signed-off-by: Shiwei Zhang <[email protected]>

specs/directory.md

@@ -49,6 +49,8 @@ Default directory paths for various operating systems at user level are specifie
 | `CONFIG`  | `$XDG_CONFIG_HOME` | `%AppData%`      | `~/Library/Application Support` |
 | `CACHE`   | `$XDG_CACHE_HOME`  | `%LocalAppData%` | `~/Library/Caches`              |
 
+On Unix, `$XDG_CONFIG_HOME` is default to `~/.config` and `$XDG_CACHE_HOME` is default to `~/.cache` if XDG environment variables are empty.
+
 There is no default `BIN` path at user level since the `notation` binary can be put anywhere as long as it in the `PATH` environment variable. Common directories on Unix/Darwin are `~/bin` and `~/.local/bin` where manual `PATH` update by users may be required.
 
 ## Structure