Update README and add contrib dir

contrib/tsn/model/transmission-gate.cc

@@ -0,0 +1,65 @@
+#include "transmission-gate.h"
+
+#include "ns3/log.h"
+#include "ns3/packet.h"
+
+namespace ns3
+{
+
+NS_LOG_COMPONENT_DEFINE("TransmissionGate");
+
+NS_OBJECT_ENSURE_REGISTERED(TransmissionGate);
+
+TypeId
+TransmissionGate::GetTypeId()
+{
+    static TypeId tid =
+        TypeId("ns3::TransmissionGate")
+            .SetParent<Object>()
+            .SetGroupName("Tsn")
+            .AddConstructor<TransmissionGate>();
+    return tid;
+}
+
+
+
+TransmissionGate::TransmissionGate()
+{
+  NS_LOG_FUNCTION(this);
+  m_state = OPEN;
+}
+
+TransmissionGate::~TransmissionGate()
+{
+    NS_LOG_FUNCTION(this);
+}
+
+bool
+TransmissionGate::IsOpen()
+{
+  NS_LOG_FUNCTION(this);
+  if(m_state == OPEN)
+  {
+    return true;
+  }
+  else
+  {
+    return false;
+  }
+}
+
+void
+TransmissionGate::Open()
+{
+    m_state = OPEN;
+}
+
+void
+TransmissionGate::Close()
+{
+    m_state = CLOSE;
+}
+
+
+
+};