added note to gradle plugin README about openApiFile parameter

Author: frantuma

modules/swagger-gradle-plugin/README.md

@@ -58,7 +58,7 @@ Parameter | Description | Required | Default
 `resourcePackages`|see [configuration property](https://github.com/swagger-api/swagger-core/wiki/Swagger-2.X---Integration-and-Configuration#configuration-properties)|false|
 `resourceClasses`|see [configuration property](https://github.com/swagger-api/swagger-core/wiki/Swagger-2.X---Integration-and-Configuration#configuration-properties)|false|
 `prettyPrint`|see [configuration property](https://github.com/swagger-api/swagger-core/wiki/Swagger-2.X---Integration-and-Configuration#configuration-properties)|false|`TRUE`
-`openApiFile`|openapi file to be merged with resolved specification, see [config](https://github.com/swagger-api/swagger-core/wiki/Swagger-2.X---Integration-and-Configuration#configuration-properties)|false|
+`openApiFile`|openapi file to be merged with resolved specification, equivalent to [config](https://github.com/swagger-api/swagger-core/wiki/Swagger-2.X---Integration-and-Configuration#configuration-properties) openAPI|false|
 `filterClass`|see [configuration property](https://github.com/swagger-api/swagger-core/wiki/Swagger-2.X---Integration-and-Configuration#configuration-properties)|false|
 `readerClass`|see [configuration property](https://github.com/swagger-api/swagger-core/wiki/Swagger-2.X---Integration-and-Configuration#configuration-properties)|false|
 `scannerClass`|see [configuration property](https://github.com/swagger-api/swagger-core/wiki/Swagger-2.X---Integration-and-Configuration#configuration-properties)|false|
@@ -68,5 +68,23 @@ Parameter | Description | Required | Default
 `modelConverterClasses`|see [configuration property](https://github.com/swagger-api/swagger-core/wiki/Swagger-2.X---Integration-and-Configuration#configuration-properties)|false|
 `contextId`|see [Context](https://github.com/swagger-api/swagger-core/wiki/Swagger-2.X---Integration-and-Configuration#context)|false|
 
-***
 
+**Note** parameter `openApiFile` corresponds to [config](https://github.com/swagger-api/swagger-core/wiki/Swagger-2.X---Integration-and-Configuration#configuration-properties) openAPI. It points to a location of a file in YAML or JSON format representing the input spec that will be merged with the resolved spec. Typically used to add Info section, or any other meta data. 
+An example of such file:
+
+```yaml
+openapi: 3.0.1
+info:
+  version: '1.0'
+  title: Swagger Pet Sample App Config File
+  description: 'This is a sample server Petstore server.  You can find out more
+    about Swagger at [http://swagger.io](http://swagger.io) or on [irc.freenode.net,
+    #swagger](http://swagger.io/irc/).  For this sample, you can use the api key
+    special-key to test the authorization filters.'
+  termsOfService: http://swagger.io/terms/
+  contact:
+    email: [email protected]
+  license:
+    name: Apache 2.0
+    url: http://www.apache.org/licenses/LICENSE-2.0.html
+```