all.do: add some hints about how to run sshuttle.

This is mostly so that people know how to find the MacOS GUI app, which was
previously rather non-obvious.

all.do

+exec >&2
 UI=
 [ "$(uname)" = "Darwin" ] && UI=ui-macos/all
 redo-ifchange sshuttle.8 $UI
 
+echo
+echo "What now?"
+[ -z "$UI" ] || echo "- Try the MacOS GUI: open ui-macos/Sshuttle*.app"
+echo "- Run sshuttle: ./sshuttle --dns -r HOSTNAME 0/0"
+echo "- Read the README: less README.md"
+echo "- Read the man page: less sshuttle.md"