[libsigcplusplus] README: Mention the move here too.

[Murray Cumming <murrayc src gnome org>]

commit 9ebd3b1bd28a21758c248106323a4b68837ad718
Author: Murray Cumming <murrayc murrayc com>
Date:   Wed Mar 14 21:03:45 2018 +0100

    README: Mention the move here too.

 README |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/README b/README
index a84eb68..8e4c0cf 100644
--- a/README
+++ b/README
@@ -1,3 +1,8 @@
+libsigc++ has now moved to
+https://github.com/libsigcplusplus/libsigcplusplus
+This git repository at git.gnome.org/browse/libsigcplusplus
+will not be updated.
+
 libsigc++ -- The Typesafe Callback Framework for C++