Added a Postman collection to the spec (#40)

* Added the Postman Collection JSON file

* Added a ‘Run in Postman’ button to the spec

* Removed the line that mentions ‘stateless’ etc…

Author: adrianparr

specification/nhs-website-content-api.yaml

@@ -61,9 +61,29 @@ info:
     ## Environments and testing
     | Environment       | Base URL                                                               |
     | ----------------- | ---------------------------------------------------------------------- |
+    | Sandbox           | `https://sandbox.api.service.nhs.uk/nhs-website-content/`              |
     | Integration test  | `https://int.api.service.nhs.uk/nhs-website-content/`                  |
     | Production        |  Not available yet                                                     |
 
+    ### Sandbox testing
+    Our [sandbox environment](https://digital.nhs.uk/developer/guides-and-documentation/testing#sandbox-testing):
+    * is for early developer testing
+    * only covers a limited set of scenarios
+    * is open access, so does not allow you to test authorisation
+
+    For details of sandbox test scenarios, or to try out the sandbox using our 'Try this API' feature, see the documentation for each endpoint.
+
+    Alternatively, you can try out the sandbox using our Postman collection:
+
+    [![Run in Postman](https://run.pstmn.io/button.svg)](https://app.getpostman.com/run-collection/222854-9766f4d9-e6b9-41ec-8949-ef3758316de0)
+
+    ### Integration testing
+    Our [integration test environment](https://digital.nhs.uk/developer/guides-and-documentation/testing#integration-testing):
+    * is for formal integration testing
+    * includes authorisation via an API Key
+         
+    For more details see [integration testing with our RESTful APIs](https://digital.nhs.uk/developer/guides-and-documentation/testing#integration-testing-with-our-restful-apis).
+
     ## Onboarding
     The onboarding journey for this API is currently in development.