Add GNU Project notice.

* JOIN-GNU: New file. Inspired by a suggestion from José E. Marchesi
on the gnu-prog-discuss mailing list.
* README: Refer to it.
* Makefile.am (EXTRA_DIST): Add it.

JOIN-GNU

@@ -0,0 +1,19 @@
+Join the GNU Project
+====================
+
+GNU gettext is part of the GNU Operating System, developed by the GNU Project.
+
+If you are the author of an awesome program and want to join us in
+writing Free (libre) Software, please consider making it an official
+GNU program and become a GNU Maintainer.  Instructions on how to do
+this are here:
+  https://www.gnu.org/help/evaluation
+We are looking forward to hacking with you!
+
+Don't have a program to contribute?  Look at all the other ways to help:
+  https://www.gnu.org/help/help.html
+
+And to learn more about Free (libre) Software in general, please read
+and share this page:
+  https://gnu.org/philosophy/free-sw.html
+

Makefile.am

@@ -1,5 +1,5 @@
 ## Makefile for the toplevel directory of GNU gettext
-## Copyright (C) 1995-2021 Free Software Foundation, Inc.
+## Copyright (C) 1995-2022 Free Software Foundation, Inc.
 ##
 ## 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
@@ -51,7 +51,8 @@ changelog_etc = \
   gnulib-local/ChangeLog.0
 
 EXTRA_DIST = \
-  $(changelog_etc) DEPENDENCIES PACKAGING HACKING ChangeLog.0 autogen.sh \
+  $(changelog_etc) DEPENDENCIES PACKAGING HACKING JOIN-GNU ChangeLog.0 \
+  autogen.sh \
   check-copyright-headers \
   build-aux/ac-help.sed build-aux/git-version-gen build-aux/texi2html \
   m4/fixautomake.m4 m4/woe32-dll.m4 \

README

@@ -62,3 +62,4 @@ DEPENDENCIES - list of prerequisite packages, to be installed before this one
 INSTALL      - general compilation and installation rules
 NEWS         - major changes in the current version
 THANKS       - list of contributors
+JOIN-GNU     - invitation to join the GNU project