add documentation for the ipv6 configuration option

Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap.conf

@@ -580,6 +580,18 @@ type = GmailMaildir
 type = IMAP
 
 
+# This option stands in the [Repository RemoteExample] section.
+#
+# Configure which address family to use for the connection. If not specified,
+# AF_UNSPEC is used as a fallback (default).
+#
+# AF_INET6:
+#ipv6 = True
+#
+# AF_INET:
+#ipv6 = False
+
+
 # These options stands in the [Repository RemoteExample] section.
 #
 # The following can fetch the account credentials via a python expression that