add oas documentation for getMI endpoint

Author: RossFaulds171

specification/api/components/documentation/getMI.md

@@ -0,0 +1,17 @@
+## Overview
+
+Use this endpoint to retrieve management or operational metrics relating to letter processing and print fulfilment.
+
+When you submit a get management information request, the endpoint will respond with a 200 (Success) response code along with the created data including a unique id for the record or an unsuccessful (4xx/5xx) response.
+
+Rate limiting applies. On excess requests, you may receive **429 Too Many Requests** (example error code(s): `NOTIFY_QUOTA`). Back off and retry later.
+
+## Sandbox test scenarios
+
+You can test the following scenarios in our sandbox environment.
+
+|Scenario|Request|Response|
+|--------|-------|--------|
+|Success|Request for successful MI record retrieval| 200 (Success) with the retrieved management information in the response|
+|Invalid Request|Invalid Request for MI record retrieval| 400 (Bad Request) with the error details in the body|
+|Unknown specification|Request for MI record retrieval for unknown spec|404 (Not Found)  with the error details in the body|