demo: document ability to auto update the copyright year

See #976

exampleSite/config/_default/config.toml

@@ -12,7 +12,7 @@ title = "Academic"
 baseurl = "/"
 
 # Enter a copyright notice to display in the site footer.
-# To display a copyright symbol, type `©`.
+# To display a copyright symbol, type `©`. For current year, type `{year}`.
 copyright = ""
 
 # Enable analytics by entering your Google Analytics tracking ID