Add release note

releasenotes/notes/qasm3-importer-0ba0691bcdeba72f.yaml

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+---
+features:
+  - |
+    The :mod:`qiskit.qasm3` package now contains a built-in, Rust-based parser for reading OpenQASM
+    3 programs into :class:`.QuantumCircuit`\ s, found at :func:`qiskit.qasm3.load_experimental` and
+    :func:`~.qasm3.loads_experimental`.  These are typically several times faster than the existing,
+    pure Python :func:`~.qasm3.load` and :func:`~.qasm3.loads` functions, which additionally require
+    ``qiskit-qasm3-import`` to be installed.
+
+    For example, we can create a 20,000-instruction entangling :class:`.QuantumCircuit`::
+
+      import numpy as np
+      import qiskit.qasm3
+      from qiskit.circuit.library import RealAmplitudes
+
+      qc = RealAmplitudes(100, reps=100, flatten=True)
+      qc = qc.assign_parameters(np.random.rand(qc.num_parameters))
+      oq3 = qiskit.qasm3.dumps(qc)
+
+    The old :func:`.qasm3.loads` took about 7.3s to load the resulting OpenQASM 3 program, whereas
+    :func:`.qasm3.loads_experimental` took under 300ms on a consumer Macbook Pro (i7, 2020)–a speedup
+    of 25x!
+
+    The supported feature set of the experimental parser is very limited in this preview version,
+    but this will expand as both the Qiskit side and `the native Rust-based parser
+    <https://github.com/Qiskit/openqasm3_parser>`__ improve.
+
+    One of our main goals with this new parser, alongside the huge speed improvements, is to provide
+    top-quality error diagnostics.  As with other parts of the parser, these are a work in progress,
+    but you'll start to see much higher quality error messages displayed when parsing invalid
+    OpenQASM 3 programs with the experimental parser.