Add service metadata file (#58)

Adds a `metadata.yaml` file to the root of the repository to describe the service's metadata and versions.

Author: RedbackThomson

metadata.yaml

@@ -0,0 +1,8 @@
+service:
+  full_name: "Amazon SageMaker"
+  short_name: "SageMaker"
+  link: "https://aws.amazon.com/sagemaker/"
+  documentation: "https://docs.aws.amazon.com/sagemaker/latest/dg/what-is.html"
+api_versions:
+  - api_version: v1alpha1
+    status: available