feat: add presigned URL upload example using oss2 SDK doc: add README.MD

examples/README.md

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+- `presigned_url_upload.py` — Generate a signed URL to upload a file to OSS securely

examples/presigned_url_upload.py

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+# examples/presigned_url_upload.py
+
+import oss2
+
+# Replace with your actual credentials and bucket info
+ACCESS_KEY_ID = '<your-access-key-id>'
+ACCESS_KEY_SECRET = '<your-access-key-secret>'
+ENDPOINT = 'https://oss-cn-hangzhou.aliyuncs.com'  # Change if needed
+BUCKET_NAME = '<your-bucket-name>'
+OBJECT_NAME = 'demo_upload_via_presigned.txt'
+EXPIRATION_SECONDS = 300  # 5 minutes
+
+# Initialize auth and bucket
+auth = oss2.Auth(ACCESS_KEY_ID, ACCESS_KEY_SECRET)
+bucket = oss2.Bucket(auth, ENDPOINT, BUCKET_NAME)
+
+# Generate presigned URL for PUT upload
+signed_url = bucket.sign_url('PUT', OBJECT_NAME, EXPIRATION_SECONDS)
+
+print("✅ Presigned URL (valid for 5 minutes):")
+print(signed_url)
+
+# Optional upload using requests
+try:
+    import requests
+    with open('example.txt', 'rb') as f:
+        response = requests.put(signed_url, data=f)
+        print("Upload status code:", response.status_code)
+except Exception as e:
+    print("Install `requests` to auto-upload, or use the URL manually.")