Update README and add contrib dir

contrib/tsn/model/clock-virtual.h

@@ -0,0 +1,52 @@
+#ifndef VIRTUALCLOCK_H
+#define VIRTUALCLOCK_H
+
+#include "ns3/object.h"
+#include "ns3/nstime.h"
+
+#include "ns3/clock.h"
+
+namespace ns3
+{
+
+class VirtualClock: public Clock
+{
+  public:
+    /**
+     * \brief Get the TypeId
+     *
+     * \return The TypeId for this class
+     */
+    static TypeId GetTypeId();
+
+    /**
+     * \brief Create a VirtualClock
+     */
+    VirtualClock();
+
+    /**
+     * Destroy a VirtualClock
+     *
+     * This is the destructor for the VirtualClock.
+     */
+    ~VirtualClock() override;
+
+    // Delete copy constructor and assignment operator to avoid misuse
+    VirtualClock& operator=(const VirtualClock&) = delete;
+    VirtualClock(const VirtualClock&) = delete;
+
+    Time GetLocalTime() override;
+    Time GetUncorrectedLocalTime() override;
+    Time GetTimeAt(Time t) override;
+    Time GetDuration(Time t) override;
+    void SetRefClock(Ptr<Clock> refClock);
+
+  protected:
+
+  private:
+    Ptr<Clock> m_refClock = nullptr;
+
+};
+
+}
+#endif /* VIRTUALCLOCK_H */