Added BUCKET_NAME & IAM details to Readme (#42)

* Added BUCKET_NAME & IAM details to Readme

* Sync

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Co-authored-by: Frank <wangfanjie@gmail.com>

Author: teriu

README.md

@@ -115,7 +115,11 @@ public,max-age=0,s-maxage=31536000,must-revalidate
 
 #### Image optimization function
 
-Create a Lambda function using the code in the `.open-next/image-optimization-function` folder, with the handler `index.mjs`. Ensure that the **arm64** architecture is used.
+Create a Lambda function using the code in the `.open-next/image-optimization-function` folder, with the handler `index.mjs`. Also, ensure that the function is configured as follows:
+
+- Set the architecture to `arm64`.
+- Set the `BUCKET_NAME` environment variable with the value being the name of the S3 bucket where the original images are stored.
+- Grant `s3:GetObject` permission.
 
 This function handles image optimization requests when the Next.js `<Image>` component is used. The [sharp](https://www.npmjs.com/package/sharp) library, which is bundled with the function, is used to convert the image. The library is compiled against the `arm64` architecture and is intended to run on AWS Lambda Arm/Graviton2 architecture. [Learn about the better cost-performance offered by AWS Graviton2 processors.](https://aws.amazon.com/blogs/aws/aws-lambda-functions-powered-by-aws-graviton2-processor-run-your-functions-on-arm-and-get-up-to-34-better-price-performance/)