Update pypolymlp example

example/NaCl-pypolymlp/README.md

@@ -0,0 +1,9 @@
+# Example to create force constants using MLPs by pypolymlp
+
+This is an example to follow the documentation
+https://phonopy.github.io/phono3py/pypolymlp.html.
+`phono3py_params.yaml` can be also generated by
+
+```bash
+% python make_phono3py_params.py ../NaCl-rd/phono3py_params_NaCl.yaml.xz
+```

example/NaCl-pypolymlp/make_phono3py_params.py

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+import sys
+
+import phono3py
+
+ph3 = phono3py.load(sys.argv[1], produce_fc=False, log_level=1)
+ph3_new = phono3py.Phono3py(
+    ph3.unitcell,
+    supercell_matrix=ph3.supercell_matrix,
+    primitive_matrix=ph3.primitive_matrix,
+)
+ph3_new.dataset = ph3.dataset
+ph3_new.nac_params = ph3.nac_params
+ph3_new.save("phono3py_params.yaml")