Improve readme

README.md

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 **N.B.** When a bucket (CR) is deleted, it checks whether the bucket is used somewhere else before proceeding with the deletion.
 
+### Environment variables
+
+- `DEFAULT_S3_ENDPOINT`: Default endpoint applied when no secret (`spec.secret`) specified
+- `DEFAULT_S3_ACCESS_KEY`: Default access key applied when no secret (`spec.secret`) specified
+- `DEFAULT_S3_SECRET_KEY`: Default secret key applied when no secret (`spec.secret`) specified
+- `DEFAULT_S3_REGION`: Default region applied when no region is specified
+
 ## Getting Started
 
 ### Prerequisites