[libsigcplusplus] NEWS: Mention that it needs C++14.

[Murray Cumming <murrayc src gnome org>]

commit 8ee64406e1fc0c5123720fc5c5e4d0ae2cabfdc4
Author: Murray Cumming <murrayc murrayc com>
Date:   Mon Mar 7 11:53:57 2016 +0100

    NEWS: Mention that it needs C++14.

 NEWS |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/NEWS b/NEWS
index 1177c32..5ed8146 100644
--- a/NEWS
+++ b/NEWS
@@ -5,6 +5,8 @@ with libsigc++-2.0. The API is not significantly different, but the
 implementation is far simpler, using variadic templates instead of generating
 C++ code from .m4 files.
 
+libsigc++-3.0 requires a compiler with C++14 support.
+
 
 2.7.1 (unstable):