add an extensions property

Signed-off-by: Doug Davis <dug@us.ibm.com>

spec.md

@@ -154,9 +154,13 @@ used instead.
 ## Extending Resources
 
 Senders of messages defined by this specification MAY include additional
-fields within the JSON objects. When adding new fields, unique prefixes
-SHOULD be used for the field names to reduce the chances of conflicts
-with future specification defined fields.
+fields within the JSON objects as extensions. For backwards compatibility
+reasons, new fields MAY be added at any location within the JSON objects,
+however, they SHOULD be placed within the `extensions` property of the JSON
+object.
+
+When adding new fields, unique prefixes SHOULD be used for the field names to
+reduce the chances of conflicts with other extensions that might be defined.
 
 Receivers of a messages defined by this specification that contain unknown
 extension fields MUST ignore those fields and MUST NOT halt processing
@@ -745,6 +749,7 @@ For success responses, the following fields are defined:
 | --- | --- | --- |
 | state* | string | Valid values are `in progress`, `succeeded`, and `failed`. While `"state": "in progress"`, the Platform SHOULD continue polling. A response with `"state": "succeeded"` or `"state": "failed"` MUST cause the Platform to cease polling. |
 | description | string | A user-facing message displayed to the Platform API client. Can be used to tell the user details about the status of the operation. If present, MUST be a non-empty string. |
+| extensions | object | A set of extension properties. |
 
 \* Fields with an asterisk are REQUIRED.
 
@@ -874,6 +879,7 @@ For success responses, the following fields are defined:
 | --- | --- | --- |
 | dashboard_url | string | The URL of a web-based management user interface for the Service Instance; we refer to this as a service dashboard. The URL MUST contain enough information for the dashboard to identify the resource being accessed (`9189kdfsk0vfnku` in the example below). Note: a Service Broker that wishes to return `dashboard_url` for a Service Instance MUST return it with the initial response to the provision request, even if the service is provisioned asynchronously. If present, MUST be a non-empty string. |
 | operation | string | For asynchronous responses, Service Brokers MAY return an identifier representing the operation. The value of this field MUST be provided by the Platform with requests to the [Last Operation](#polling-last-operation) endpoint in a percent-encoded query parameter. If present, MUST be a non-empty string. |
+| extensions | object | A set of extension properties. |
 
 ```
 {
@@ -1031,6 +1037,7 @@ For success responses, the following fields are defined:
 | Response Field | Type | Description |
 | --- | --- | --- |
 | operation | string | For asynchronous responses, Service Brokers MAY return an identifier representing the operation. The value of this field MUST be provided by the Platform with requests to the [Last Operation](#polling-last-operation) endpoint in a percent-encoded query parameter. If present, MUST be a non-empty string. |
+| extensions | object | A set of extension properties. |
 
 ```
 {
@@ -1232,6 +1239,7 @@ For success responses, the following fields are defined:
 | syslog_drain_url | string | A URL to which logs MUST be streamed. `"requires":["syslog_drain"]` MUST be declared in the [Catalog](#catalog-management) endpoint or the Platform MUST consider the response invalid. |
 | route_service_url | string | A URL to which the Platform MUST proxy requests for the address sent with `bind_resource.route` in the request body. `"requires":["route_forwarding"]` MUST be declared in the [Catalog](#catalog-management) endpoint or the Platform can consider the response invalid. |
 | volume_mounts | array of [VolumeMount](#volume-mount-object) objects | An array of configuration for remote storage devices to be mounted into an application container filesystem. `"requires":["volume_mount"]` MUST be declared in the [Catalog](#catalog-management) endpoint or the Platform can consider the response invalid. |
+| extensions | object | A set of extension properties. |
 
 ##### Volume Mount Object