chore: add README of Clifford simulator

provided by Yanglin Zhang, to be integrated into UserGuide in future version

src/qfvm_clifford/README.md

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+# Clifford Simulator
+
+Usage of Clifford simulator in ``pyquafu`` is very similar with the default simulator, but applies only to circuits with a limited gate set.
+
+## Supported Gates
+
+### x
+
+The Pauli X gate. The bit flip gate.
+
+### y
+
+The Pauli Y gate.
+
+### z
+
+The Pauli Z gate. The phase flip gate
+
+### h
+
+The Hadamard gate. Swap the X and Z axes. Alternate name: `h_xz`
+
+### h_yz
+
+The variant of the Hadamard gate that swaps the Y and Z axes (instead of X and Z)
+
+### s
+
+Principal square root of Z gate. Phases the amplitue of |1> by i. Alternate name: `sqrt_z`
+
+### s_dag
+
+Adjoint of the principal square root of z gate. Phases the amplitube of |1> by -i. Alternate name: `sqrt_z_dag`
+
+### cnot
+
+The Z-controlled X gate. Applies an X gate to the target if the control is in the |1> state. Equivalently: negates the amplitude of the |1>|-> state. The first qubit is called the control, and the second qubit is the target. Alternate name: `cx`, `zcx`
+
+### swap
+
+Swap two qubits.
+
+### measure
+
+Z-basis measurement. Projects each target qubit into |0> or |1> and reports its value (false=|0>, true=|1>).
+
+### reset
+
+Z-basis reset. Forces each target qubit into the |0> state by silently measuring it in the Z basis and applying an X gate if it ended up in the |1> state.
+
+## Simple Example
+
+```python
+from quafu import QuantumCircuit, simulate
+
+qc = QuantumCircuit(4, 4)
+qc.x(0)
+qc.x(1)
+qc.measure([1, 2], [1, 0])
+qc.measure([3, 0], [2, 3])
+
+result = simulate(qc=qc, shots=10, simulator="qfvm_clifford", output="count_dict")
+counts = result.count
+
+# {"0101": 10}
+print(counts)
+```