Add SPDX identifier field to license object, fixes OAI#1599

Author: MikeRalphson

versions/3.1.0.md

@@ -295,7 +295,8 @@ License information for the exposed API.
 Field Name | Type | Description
 ---|:---:|---
 <a name="licenseName"></a>name | `string` | **REQUIRED**. The license name used for the API.
-<a name="licenseUrl"></a>url | `string` | A URL to the license used for the API. MUST be in the format of a URL.
+<a name="licenseIdentifier"></a>identifier | `string` | An [SPDX](https://spdx.org/spdx-specification-21-web-version#h.jxpfx0ykyb60) license expression for the API. The `identifier` field is mutually exclusive of the `url` field.
+<a name="licenseUrl"></a>url | `string` | A URL to the license used for the API. MUST be in the format of a URL. The `url` field is mutually exclusive of the `identifier` field.
 
 This object MAY be extended with [Specification Extensions](#specificationExtensions).
 
@@ -304,13 +305,13 @@ This object MAY be extended with [Specification Extensions](#specificationExtens
 ```json
 {
   "name": "Apache 2.0",
-  "url": "https://www.apache.org/licenses/LICENSE-2.0.html"
+  "identifier": "Apache-2.0"
 }
 ```
 
 ```yaml
 name: Apache 2.0
-url: https://www.apache.org/licenses/LICENSE-2.0.html
+identifier: Apache-2.0
 ```
 
 #### <a name="serverObject"></a>Server Object