Example code for Store instruction (#12982)

* Example code for Store function

* update shortened line

* Update qiskit/circuit/__init__.py

---------

Co-authored-by: Luciano Bello <bel@zurich.ibm.com>
Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>

qiskit/circuit/__init__.py

@@ -579,6 +579,16 @@
 :class:`Qubit` nor :class:`Clbit` operands, but has an explicit :attr:`~Store.lvalue` and
 :attr:`~Store.rvalue`.
 
+For example, to determine the parity of a bitstring ``cr`` and store it in another register ``creg``,
+the :class:`Store` instruction can be used in the following way::
+
+    parity = expr.lift(cr[0])
+    for i in range(1,n):
+        parity = expr.bit_xor(cr[i], parity)
+    qc.store(creg[0], parity)
+
+
+
 .. autoclass:: Store
     :show-inheritance:
     :members: