update docs on anonymous s3 requests

docs/src/sources-files.md

@@ -21,9 +21,17 @@ By default, the credentials try for default profile or environment variables. En
 - `AWS_SECRET_ACCESS_KEY`
 - `AWS_SESSION_TOKEN`
 - `AWS_PROFILE` (to specify profile instead of access key variables)
+- `AWS_REGION`
+
+Note: `AWS_REGION` must match the region of the given bucket in the S3 URI.
 
 For more information on AWS credentials, see [documentation](https://docs.aws.amazon.com/sdkref/latest/guide/creds-config-files.html).
 
+### Anonymous credentials
+
+To send requests anonymously for publicly available buckets, set the environment variable `AWS_NO_CREDENTIALS=1`.
+Note that you still need to set `AWS_REGION` to the correct region.
+
 Example configuration:
 
 ```yaml