Call out that extra fields in requests and responses are allowed

This change will make it clear that request and responses that
have HTTP bodies MAY have additional fields specified by the
sender.

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

spec.md

@@ -349,6 +349,8 @@ It is therefore RECOMMENDED that implementations avoid such strings.
 
 \* Fields with an asterisk are REQUIRED.
 
+Service Brokers MAY include additional fields within the response.
+
 ##### Service Objects
 
 | Response field | Type | Description |
@@ -719,6 +721,8 @@ For success responses, the following fields are defined:
 
 \* Fields with an asterisk are REQUIRED.
 
+Service Brokers MAY include additional fields within the response.
+
 ```
 {
   "state": "in progress",
@@ -783,6 +787,8 @@ The following HTTP Headers are defined for this operation:
 
 \* Fields with an asterisk are REQUIRED.
 
+Platforms MAY include additional fields within the request.
+
 ```
 {
   "service_id": "service-id-here",
@@ -849,6 +855,8 @@ For success responses, the following fields are defined:
 }
 ```
 
+Service Brokers MAY include additional fields within the response.
+
 ## Updating a Service Instance
 
 By implementing this endpoint, Service Broker authors can enable users to
@@ -903,6 +911,8 @@ The following HTTP Headers are defined for this operation:
 
 \* Fields with an asterisk are REQUIRED.
 
+Platforms MAY include additional fields within the request.
+
 ##### Previous Values Object
 
 | Request Field | Type | Description |
@@ -997,6 +1007,8 @@ For success responses, the following fields are defined:
 }
 ```
 
+Service Brokers MAY include additional fields within the response.
+
 
 ## Binding
 
@@ -1104,6 +1116,8 @@ The following HTTP Headers are defined for this operation:
 
 \* Fields with an asterisk are REQUIRED.
 
+Platforms MAY include additional fields within the request.
+
 ##### Bind Resource Object
 
 The `bind_resource` object contains Platform specific information related to
@@ -1188,6 +1202,8 @@ For success responses, the following fields are defined:
 | 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-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. |
 
+Service Brokers MAY include additional fields within the response.
+
 ##### Volume Mounts Object
 
 | Response Field | Type | Description |
@@ -1200,6 +1216,8 @@ For success responses, the following fields are defined:
 
 \* Fields with an asterisk are REQUIRED.
 
+Service Brokers MAY include additional fields within the response.
+
 ##### Device Object
 
 Currently only shared devices are supported; a distributed file system which
@@ -1212,6 +1230,8 @@ can be mounted on all app instances simultaneously.
 
 \* Fields with an asterisk are REQUIRED.
 
+Service Brokers MAY include additional fields within the response.
+
 ```
 {
   "credentials": {
@@ -1308,6 +1328,8 @@ Platform MUST continue to remember the Service Binding.
 
 For a success response, the expected response body is `{}`.
 
+Service Brokers MAY include additional fields within the response.
+
 ## Deprovisioning
 
 When a Service Broker receives a deprovision request from a Platform, it MUST
@@ -1379,6 +1401,8 @@ For success responses, the following fields are defined:
 | --- | --- | --- |
 | 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. |
 
+Service Brokers MAY include additional fields within the response.
+
 ```
 {
   "operation": "task_10"