chore: update spec doc with referenceTaskIds (#621)

Author: swapydapy

docs/specification.md

@@ -5,7 +5,7 @@ hide:
 
 # Agent2Agent (A2A) Protocol Specification
 
-**Version:** `0.2.0`
+**Version:** `0.2.1`
 
 ## 1. Introduction
 
@@ -488,27 +488,29 @@ interface Message {
   // Keys SHOULD be strings; values can be any valid JSON type.
   // Useful for timestamps, source identifiers, language codes, etc.
   metadata?: Record<string, any>;
-  //message identifier created by the message creator
+  // List of tasks referenced as contextual hint by this message.
+  referenceTaskIds?: string[];
+  // message identifier created by the message creator
   messageId: string;
-  //task identifier the current message is related to
+  // task identifier the current message is related to
   taskId?: string
-  //Context identifier the message is associated with
+  // Context identifier the message is associated with
   contextId?: string;
-  //type discriminator
+  // type discriminator
   kind: 'message'
 }
 ```
 
-| Field Name | Type                            | Required | Description                                                                      |
-| :--------- | :------------------------------ | :------- | :------------------------------------------------------------------------------- |
-| `role`     | `"user"` \| `"agent"`           | Yes      | Indicates the sender: `"user"` (from A2A Client) or `"agent"` (from A2A Server). |
-| `parts`    | [`Part[]`](#65-part-union-type) | Yes      | Array of content parts. Must contain at least one part.                          |
-| `metadata` | `Record<string, any>`                        | No       | Arbitrary key-value metadata associated with this message.                       |
-| `messageId` | `string`                       | Yes      | Message identifier generated by the message sender                         |
-| `taskId`    | `string`                       | No       | Task identifier the current message is related to                         |
-| `contextId` | `string`                       | No       | Context identifier the message is associated with                         |
-| `kind`  | `"message"`                    | Yes      | Type discriminator, literal value                                               |
-
+| Field Name         | Type                            | Required | Description                                                                      |
+| :----------------- | :------------------------------ | :------- | :------------------------------------------------------------------------------- |
+| `role`             | `"user"` \| `"agent"`           | Yes      | Indicates the sender: `"user"` (from A2A Client) or `"agent"` (from A2A Server). |
+| `parts`            | [`Part[]`](#65-part-union-type) | Yes      | Array of content parts. Must contain at least one part.                          |
+| `metadata`         | `Record<string, any>`           | No       | Arbitrary key-value metadata associated with this message.                       |
+| `referenceTaskIds` | `string[]`                      | No       | List of tasks referenced as contextual hint by this message.                     |
+| `messageId`        | `string`                        | Yes      | Message identifier generated by the message sender                               |
+| `taskId`           | `string`                        | No       | Task identifier the current message is related to                                |
+| `contextId`        | `string`                        | No       | Context identifier the message is associated with                                |
+| `kind`             | `"message"`                     | Yes      | Type discriminator, literal value                                                |
 
 ### 6.5. `Part` Union Type