add documentation about using environment variables in config file

README.md

@@ -34,9 +34,16 @@ By default, the [XDG Base Directory Specification] and [xdg-user-dirs] are used
 
 You can overwrite this by writing a configuration file for `sun` in `${XDG_CONFIG_HOME:-~/.config}/sun/config.scfg` with the following content:
 
- ```scfg
+```scfg
 output_dir /home/winniethepooh/screenshots
- ```
+```
+
+You can use environment variables this way:
+
+```scfg
+output_dir $(HOME)/screenshots
+```
+
 
 ## About