add versioning for Agency and Policy API

agency/README.md

@@ -7,6 +7,7 @@ This specification contains a collection of RESTful APIs used to specify the dig
 
 ## Table of Contents
 
+* [General Information](#general-information)
 * [Authorization](#authorization)
 * [Timestamps](#timestamps)
 * [Vehicles](#vehicles)
@@ -18,6 +19,18 @@ This specification contains a collection of RESTful APIs used to specify the dig
 * [Enum definitions](#enum-definitions)
 * [Responses](#responses)
 
+## General information
+
+The following information applies to all `agency` API endpoints. Details on providing authorization to endpoints is specified in the [Authorization](#authorization) section.
+
+### Versioning
+
+`agency` APIs must handle requests for specific versions of the specification from clients.
+
+Versioning must be implemented as specified in the [`Provider API versioning section`][Provider/versioning].
+
+Since versioning was not added until the 0.4.1 release, if the Accept header is not set as specified above, the Agency API must respond as if version 0.3 was requested.
+
 ## Authorization
 
 When making requests, the Agency API expects `provider_id` to be part of the claims in a [JWT](https://jwt.io/)  `access_token` in the `Authorization` header, in the form `Authorization: Bearer <access_token>`. The token issuance, expiration and revocation policies are at the discretion of the Agency.
@@ -299,3 +312,7 @@ A vehicle may have one or more values from the `propulsion`, depending on the nu
 | `error`             | String   | Error message string   |
 | `error_description` | String   | Human readable error description (can be localized) |
 | `error_details`     | String[] | Array of error details |
+
+[Top][toc]
+
+[provider/versioning]: /provider/README.md#versioning