[libchamplain] Add a comment to AUTHORS that the file is no longer updated

[Jiří Techet <jiritechet src gnome org>]

commit fd8bec942307c1c731df98eee22c8961be1eb7d5
Author: Jiří Techet <techet gmail com>
Date:   Mon Feb 18 16:29:07 2019 +0100

    Add a comment to AUTHORS that the file is no longer updated

 AUTHORS | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)
---
diff --git a/AUTHORS b/AUTHORS
index 2497f14..763dc6c 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -1,3 +1,11 @@
+This file is no longer updated. Use
+
+git log --format="%an <%ae>" | sort -u
+
+for the complete list of contributors.
+
+---
+
 Pierre-Luc Beaudoin <pierre-luc pierlux com>
 Jonathon Jongsma <jonathon quotidian org>
 Lorenzo Masini <lorenxo86 gmail com>
@@ -41,4 +49,3 @@ Yaakov Selkowitz <yselkowitz users sourceforge net>
 Nirbheek Chauhan <nirbheek gentoo org>
 William Jon McCann <jmccann redhat com>
 Zeeshan Ali <zeeshanak gnome org>
-