man page: Document new options file location

The options file can now be stored under $XDG_CONFIG_HOME, so the man
page has been updated.

Author: cherryman

tup.1

@@ -243,6 +243,7 @@ The options are read in the following precedence order:
 .nf
   command-line overrides (eg: -j flag passed to 'tup')
   \&.tup/options file
+  ${XDG_CONFIG_HOME:-$HOME/.config}/tup/config
   ~/.tupoptions file
   /etc/tup/options file
   tup's compiled in defaults