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.in (SOURCE_FILES): Add it.
diff --git a/ChangeLog b/ChangeLog
index ca0b68a..658fb63 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2022-05-22  Bruno Haible  <bruno@clisp.org>
+
+	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.in (SOURCE_FILES): Add it.
+
 2022-05-15  Bruno Haible  <bruno@clisp.org>
 
 	Switch to libtool 2.4.7.
diff --git a/JOIN-GNU b/JOIN-GNU
new file mode 100644
index 0000000..559b4e8
--- /dev/null
+++ b/JOIN-GNU
@@ -0,0 +1,19 @@
+Join the GNU Project
+====================
+
+GNU libiconv 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
+
diff --git a/Makefile.in b/Makefile.in
index 1ff4928..4fb5eea 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -147,7 +147,8 @@
 
 # List of source files.
 SOURCE_FILES = \
-  README DEPENDENCIES INSTALL.windows HACKING AUTHORS NEWS DESIGN NOTES THANKS \
+  README DEPENDENCIES INSTALL.windows HACKING AUTHORS NEWS JOIN-GNU \
+  DESIGN NOTES THANKS \
   ChangeLog Makefile.devel configure.ac Makefile.in \
   include/export.h include/iconv.h.in \
   gnulib-local/lib/alloca.in.h \
diff --git a/README b/README
index e7375cb..067d915 100644
--- a/README
+++ b/README
@@ -169,5 +169,10 @@
   - in the bug tracker at <https://savannah.gnu.org/projects/libiconv>
   - or by email to <bug-gnu-libiconv@gnu.org>.
 
+Join the GNU project
+--------------------
+
+See file JOIN-GNU.
+
 
 Bruno Haible <bruno@clisp.org>