docs: add info about IRSA for S3 authn

Signed-off-by: Artem Kajalainen <artem@iki.fi>

docs/s3.md

@@ -7,6 +7,8 @@ AWS SDK has different options for credentials. We support:
 1. Providing [Access Keys](https://docs.aws.amazon.com/general/latest/gr/aws-sec-cred-types.html#access-keys-and-secret-access-keys).
 2. Use IAM roles for Amazon EC2
 3. Use IAM roles for tasks if your application uses an ECS task definition
+4. Utilizing [IAM roles](https://docs.aws.amazon.com/eks/latest/userguide/iam-roles-for-service-accounts.html) for service accounts (IRSA) if you operate SFTPGo atop AWS EKS.
+5. Assuming specific IAM role by setting its ARN.
 
 So, you need to provide access keys to activate option 1, or leave them blank to use the other ways to specify credentials.