Added reference to no payload in schema with last_updated field

Signed-off-by: Michael Schnuerle <1285077+schnuerle@users.noreply.github.com>

Author: schnuerle

policy/README.md

@@ -166,7 +166,7 @@ _Query Parameters:_
 | `policy_id`         | UUID      | Optional    | If provided, returns one policy object with the matching UUID; default is to return all policy objects.                       |
 | `start_date` | [timestamp][ts] | Optional    | Beginning date of the queried time range; the default value is the request time |
 | `end_date`   | [timestamp][ts] | Optional    | Ending date of the queried time range; the default value is null, which captures all policies that are effective in the future|
-| `last_updated`   | Boolean | Optional    | If true, the endpoint only returns two fields: `version`, `last_updated`. Useful to quickly check when any data in the file has last been changed, without downloading the entire Policy payload. |
+| `last_updated`   | Boolean | Optional    | If true, the endpoint only returns two fields: `version`, `last_updated`. Useful to quickly check when any data in the file has last been changed, without downloading the entire Policy payload. See the [Schema](#schema) section for an example. |
 | `active_only`   | Boolean | Optional    | If true, return only the current active and future policies, not the retired/previous policies. Any policy that is a prev_policies would not be returned. However, the array of prev_policies still will be returned for reference as part of any relevant active policy. Useful to reduce the Policy payload size for use cases where you do not need to know the previous policy details. |
 
 `start_date` and `end_date` are only considered when no `id` parameter is provided. They should return any policy whose effectiveness overlaps with or is contained with this range. Suppose there's a policy with a `start_date` of 1/1/21 and `end_date` of 1/31/21. Assuming an `end_date` that is null, 12/1/20 and 1/5/21 `start_dates` will return the policy, but 2/10/21 wouldn't. Assuming a `start_date` parameter of say, 11/1/20, then an `end_date` of 12/1/20 wouldn't return the policy, but 1/5/21 and 2/10/21 would. Lastly, a `start_date` of 1/5/21 and `end_date` of 1/6/21 would also return the policy. Please note also that while dates in the format MM:DD:YY are being used here, `start_date` and `end_date` must be numbers representing milliseconds since the Unix epoch time.
@@ -295,6 +295,8 @@ Response bodies must be a `UTF-8` encoded JSON object and must minimally include
 }
 ```
 
+Note that `data` payload will not be returned if the `last_updated` query string parameter is set to true. 
+
 ### Data Schema
 
 See the [Endpoints](#endpoints) below for information on their specific schema, and the [`mds-openapi`](https://github.com/openmobilityfoundation/mds-openapi) repository for full details and interactive documentation.