Update CHANGELOG for #4257

CHANGELOG.md

@@ -6,6 +6,7 @@ ENHANCEMENTS:
 * data-source/aws_ecs_cluster Return error on multiple clusters [GH-4286]
 * resource/aws_api_gateway_rest_api: Add `policy` argument [GH-4211]
 * resource/aws_instance: Add `credit_specification` argument (e.g. t2.unlimited support) [GH-2619]
+* resource/aws_launch_configuration: Add `user_data_base64` argument [GH-4257]
 * resource/aws_s3_bucket: Add support for `ONEZONE_IA` storage class [GH-4287]
 * resource/aws_s3_bucket_object: Add support for `ONEZONE_IA` storage class [GH-4287]
 * resource/aws_spot_instance_request: Add `valid_from` and `valid_until` arguments [GH-4018]