[libsigcplusplus] Update NEWS

[Murray Cumming <murrayc src gnome org>]

commit 878ede3abce6fb730c5c01d3435148cd6d48b5ae
Author: Murray Cumming <murrayc murrayc com>
Date:   Mon Mar 7 11:29:45 2016 +0100

    Update NEWS

 NEWS |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)
---
diff --git a/NEWS b/NEWS
index 13518a8..1177c32 100644
--- a/NEWS
+++ b/NEWS
@@ -1,3 +1,11 @@
+2.99.1 (unstable):
+
+This is the first release of the libsigc++-3.0 API, which installs in parallel
+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.
+
+
 2.7.1 (unstable):
 
 * signal: Add a moving connect() method, taking an rvalue reference