README.md: update gnu directory description

(At this point in main we have just LGPL libdialog and dialog, and GPL
diff3, left in gnu/.)

Reported by: bapt
MFC after: 3 days
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 266cbbfc65263a3b8bc7c95dd3d589aab7cefe7f)
(cherry picked from commit 799a0d6969d111fd1be995cde599d8d3b1479777)