update mailing list; mention Matrix chat

Signed-off-by: Yukai Chou <[email protected]>

Author: muzimuzhi

doc/latex/pgfplots/pgfplots.install.tex

@@ -297,10 +297,11 @@ \subsection{Problems with Language Settings and Active Characters}
 \subsection{Other Problems}
 
 Please read the issue list at \url{https://github.com/pgf-tikz/pgfplots/issues}
-or the mailing list at \url{https://sourceforge.net/projects/pgfplots/support}.
-Perhaps someone has
-also encountered your problem before, and maybe he came up with a solution.
-
-Please open a new issue if you have a different problem. In
-case it is necessary to contact the authors directly, consider the addresses
-shown on the title page of this document.
+or the mailing list \url{https://tug.org/mailman/listinfo/pgf-tikz}. Perhaps
+someone has also encountered your problem before, and maybe he came up with a
+solution.
+
+Please open a new issue if you have a different problem. There is also a chat
+on the Matrix network at
+\href{https://matrix.to/#/#pgf-tikz:matrix.org}{#pgf-tikz:matrix.org}
+(\href{https://view.matrix.org/room/!NuxCISwYQJuyWwNsEI:matrix.org/}{read-only version}).