update_project_metadataの使用例追記

Author: HiroKb

README.md

@@ -2527,6 +2527,19 @@ Copy a project.
 project_id = client.copy_project(project_id="YOUR_PROJECT_ID")
 ```
 
+### Update Project Metadata
+
+Update a project metadata.
+
+```python
+project_metadata_id = client.update_project_metadata(
+  project_id="YOUR_PROJECT_ID",
+  external_code_1="ExternalCode1",
+  external_code_2="ExternalCode2",
+  external_code_3=None
+)
+```
+
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