maint: describe policy on copyright year number ranges

* README: Mention coreutils' long-standing policy on use of M-N
ranges in copyright year lists.  Requested by Richard Stallman.

README

@@ -44,6 +44,9 @@ this distribution:
 * help2man 1.33   <ftp://ftp.gnu.org/gnu/help2man/help2man-1.33.1.tar.gz>
 * Texinfo 4.7     <ftp://ftp.gnu.org/gnu/texinfo/texinfo-4.7.tar.gz>
 
+For any copyright year range specified as YYYY-ZZZZ in this package
+note that the range specifies every single year in that closed interval.
+
 Please report bugs to <bug-gnu-utils@gnu.org>.
 
 -----
@@ -53,15 +56,9 @@ Inc.
 
 This file is part of GNU Diffutils.
 
-This program is free software: you can redistribute it and/or modify
-it under the terms of the GNU General Public License as published by
-the Free Software Foundation, either version 3 of the License, or
-(at your option) any later version.
-
-This program is distributed in the hope that it will be useful,
-but WITHOUT ANY WARRANTY; without even the implied warranty of
-MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-GNU General Public License for more details.
-
-You should have received a copy of the GNU General Public License
-along with this program.  If not, see <http://www.gnu.org/licenses/>.
+Permission is granted to copy, distribute and/or modify this document
+under the terms of the GNU Free Documentation License, Version 1.3 or
+any later version published by the Free Software Foundation; with no
+Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
+Texts.  A copy of the license is included in the ``GNU Free
+Documentation License'' file as part of this distribution.