Add Link spec

SPEC.md

@@ -97,6 +97,10 @@ 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. The handler can use it to associate resources from the caller to the
+operations. You can pass multiple `Nexus-Link` header to send multiple links. See the [Links][#Links] section for more
+details.
+
 #### Request Body
 
 The body may contain arbitrary data. Headers should specify content type and encoding.
@@ -111,11 +115,13 @@ 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 to associate
+  resources to this operation.
 
   **Headers**:
 
   - `Content-Type: application/json`
+  - `Nexus-Link: <url>; type="..."`
 
   **Body**: A JSON serialized [`OperationInfo`](#operationinfo) object.
 
@@ -264,6 +270,31 @@ following predefined error codes.
 | `UNAVAILABLE`        | 503         | The service is currently unavailable.                                                                                            |
 | `UPSTREAM_TIMEOUT`   | 520         | Used by gateways to report that a request to an upstream server has timed out.                                                   |
 
+## 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.
+```
+
+The `Nexus-Link` header field provides a means for serializing one or more `Link` objects in HTTP headers. This header
+is encoded the same way as the HTTP header `Link` described [here](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Link).
+
+Handlers and callers can specify links in different Nexus requests/responses to associate an operation with arbitrary
+resources.
+
 ## General Purpose Headers
 
 ### `Request-Timeout`