Update README and add contrib dir

contrib/tsn/model/clock-fix-precision.cc

@@ -0,0 +1,83 @@
+#include "clock-fix-precision.h"
+
+#include "ns3/log.h"
+#include "ns3/simulator.h"
+#include "ns3/uinteger.h"
+#include <ns3/double.h>
+
+#include "ns3/clock.h"
+
+namespace ns3
+{
+
+NS_LOG_COMPONENT_DEFINE("FixPrecisionClock");
+
+NS_OBJECT_ENSURE_REGISTERED(FixPrecisionClock);
+
+TypeId
+FixPrecisionClock::GetTypeId()
+{
+    static TypeId tid =
+        TypeId("ns3::FixPrecisionClock")
+            .SetParent<Clock>()
+            .SetGroupName("tsn")
+            .AddConstructor<FixPrecisionClock>()
+            .AddAttribute("Precision",
+                          "Fix precision offset according to the reference clock (i.e. GM)",
+                          TimeValue(Time(0)),
+                          MakeTimeAccessor(&FixPrecisionClock::m_precision),
+                          MakeTimeChecker());
+    return tid;
+}
+
+
+FixPrecisionClock::FixPrecisionClock()
+{
+    NS_LOG_FUNCTION(this);
+}
+
+FixPrecisionClock::~FixPrecisionClock()
+{
+    NS_LOG_FUNCTION(this);
+}
+
+void
+FixPrecisionClock::SetRefClock(Ptr<Clock> refClock)
+{
+  NS_LOG_FUNCTION(this);
+  m_refClock = refClock;
+}
+
+Time
+FixPrecisionClock::GetLocalTime()
+{
+  NS_LOG_FUNCTION(this);
+  return GetUncorrectedLocalTime() + m_precision;
+}
+
+Time
+FixPrecisionClock::GetUncorrectedLocalTime()
+{
+  NS_LOG_FUNCTION(this);
+
+  NS_ASSERT_MSG(m_refClock != nullptr,
+            "Reference clock no set using void VirtualClock::SetRefClock(Ptr<Clock> refClock)");
+  return m_refClock->GetUncorrectedLocalTime();
+}
+
+Time
+FixPrecisionClock::GetTimeAt(Time t)
+{
+  NS_LOG_FUNCTION(this);
+  return m_precision + m_refClock->GetTimeAt(t);
+}
+
+Time
+FixPrecisionClock::GetDuration(Time d)
+{
+  NS_LOG_FUNCTION(this);
+  return  m_refClock->GetDuration(d);
+}
+
+
+}