Update README and add contrib dir

contrib/tsn/model/clock-constant-drift.h

@@ -0,0 +1,56 @@
+#ifndef CONSTANTDRIFTCLOCK_H
+#define CONSTANTDRIFTCLOCK_H
+
+#include "ns3/object.h"
+#include "ns3/nstime.h"
+
+#include "ns3/clock.h"
+
+namespace ns3
+{
+
+class ConstantDriftClock: public Clock
+{
+  public:
+    /**
+     * \brief Get the TypeId
+     *
+     * \return The TypeId for this class
+     */
+    static TypeId GetTypeId();
+
+    /**
+     * \brief Create a ConstantDriftClock
+     */
+    ConstantDriftClock();
+
+    /**
+     * Destroy a ConstantDriftClock
+     *
+     * This is the destructor for the ConstantDriftClock.
+     */
+    ~ConstantDriftClock() override;
+
+    // Delete copy constructor and assignment operator to avoid misuse
+    ConstantDriftClock& operator=(const ConstantDriftClock&) = delete;
+    ConstantDriftClock(const ConstantDriftClock&) = delete;
+
+    Time GetLocalTime() override;
+    Time GetUncorrectedLocalTime() override;
+    Time GetTimeAt(Time t) override;
+    Time GetDuration(Time t) override;
+    void UpdateTime() override;
+
+  protected:
+
+  private:
+    Time m_initialOffset;
+    bool m_IsInitialised = false;
+    Time m_lastUpdateTime;
+    Time m_lastUpdateTimeValue;
+    double m_driftRate;
+
+};
+
+}
+#endif /* CONSTANTDRIFTCLOCK_H */