add translation credits

Insert link to Ring's Transifex project page since the list of
translation contributors can be found there.

Change-Id: I101d0e917b2ad997f69f4cdda461859835dac02d
Tuleap: #134
diff --git a/src/dialogs.c b/src/dialogs.c
index fd4e943..25cfee1 100644
--- a/src/dialogs.c
+++ b/src/dialogs.c
@@ -116,6 +116,7 @@
         "website", "http://www.ring.cx/",
         "website-label", "www.ring.cx",
         "artists", artists,
+        "translator-credits", "https://www.transifex.com/savoirfairelinux/ring",
         NULL
     );