Add Link spec

SPEC.md

@@ -72,6 +72,25 @@ properties:
       Describes the current state of an operation.
 ```
 
+### Links
+
+The `Link` object represents a link to which the receiver can use and associate the operation.
+The object MUST adhere to the given schema:
+
+```yaml
+type: object
+properties:
+  url:
+    type: url
+    description: |
+      The URL the link is pointing to.
+
+  type:
+    type: string
+    description: |
+      Type provides additional information about the URL that can be used to parse it.
+```
+
 ## Endpoint Descriptions
 
 ### Start Operation
@@ -97,6 +116,10 @@ Headers that start with the `Nexus-Callback-` prefix are expected to be attached
 the handler. The callback request must strip away the `Nexus-Callback-` prefix. E.g if a Start Operation request
 includes a `Nexus-Callback-Token: some-token` header, the callback request would include a `Token: some-token` header.
 
+The header `Nexus-Link` contains a `Link` object encoded the same way as the HTTP header `Link` described
+[here](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Link). The handler can use it to associate
+the operations to the links sent by the caller. You can pass multiple `Nexus-Link` header to send multiple links.
+
 #### Request Body
 
 The body may contain arbitrary data. Headers should specify content type and encoding.
@@ -111,11 +134,13 @@ The body may contain arbitrary data. Headers should specify content type and enc
 
   **Body**: Arbitrary data conveying the operation's result. Headers should specify content type and encoding.
 
-- `201 Created`: Operation was started and will complete asynchronously.
+- `201 Created`: Operation was started and will complete asynchronously. It may return `Nexus-Link` headers that
+  contain information about the operations done by the handler.
 
   **Headers**:
 
   - `Content-Type: application/json`
+  - `Nexus-Link: <url>; type="..."`
 
   **Body**: A JSON serialized [`OperationInfo`](#operationinfo) object.