Add maintenance note.

README.md

diff --git a/README.md b/README.md
index 8f2d2dd..7258ca7 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,8 @@
+> NOTE: This repository is no longer maintained. The Netstack code will continue
+> to be updated and maintained as part of
+> [gVisor](http://www.github.com/google/gvisor/tree/go), which now also
+> maintains a branch that is useable with standard Go tools.
+
 # Netstack
 
 Netstack is a network stack written in Go.