added AWS credentials file example

examples/s3-to-s3-cross-account/.header.md

@@ -3,3 +3,16 @@
 This example demonstrates how to create a S3 to S3 replication scenario across AWS Accounts using the AWS DataSync module.
 
 ![AWS Datasync S3 to S3 Cross Account](./datasync-examples-cross-account.png)
+
+In this example, the default source and destination account AWS CLI profiles are named as `source-account` and `destination-account` in [variables.tf](./variables.tf). Add the following text to the shared credentials file. Replace the sample values with the credentials you copied. Review [Authenticating with short-term credentials for the AWS CLI](https://docs.aws.amazon.com/cli/latest/userguide/cli-authentication-short-term.html) documentation for more information.
+```bash
+[source-account] 
+aws_access_key_id = EXAMPLE 
+aws_secret_access_key = EXAMPLEKEY
+aws_session_token = LONGSTRINGEXAMPLE
+
+[destination-account]
+aws_access_key_id = EXAMPLE 
+aws_secret_access_key = EXAMPLEKEY
+aws_session_token = LONGSTRINGEXAMPLE
+```