Make usage of -D more explicit in example

Author: ernoc

README.md

@@ -507,7 +507,7 @@ and `config.json` contains the following as an example:
 ```
 
 Supported config options can be different per language. Running `config-help -l {lang}` will show available options.
-**These options are applied via configuration file (e.g. config.json) or by passing them with `-D{optionName}={optionValue}`**. (If `-D{optionName}` does not work, please open a [ticket](https://github.com/swagger-api/swagger-codegen/issues/new) and we'll look into it)
+**These options are applied via configuration file (e.g. config.json) or by passing them with `java -jar swagger-codegen-cli.jar -D{optionName}={optionValue}`**. (If `-D{optionName}` does not work, please open a [ticket](https://github.com/swagger-api/swagger-codegen/issues/new) and we'll look into it)
 
 ```sh
 java -jar modules/swagger-codegen-cli/target/swagger-codegen-cli.jar config-help -l java
@@ -532,7 +532,7 @@ CONFIG OPTIONS
 	    retrofit - HTTP client: OkHttp 2.4.0. JSON processing: Gson 2.3.1 (Retrofit 1.9.0)
         retrofit2 - HTTP client: OkHttp 2.5.0. JSON processing: Gson 2.4 (Retrofit 2.0.0-beta2)
         google-api-client - HTTP client: google-api-client 1.23.0. JSON processing: Jackson 2.8.9
-        rest-assured - HTTP client: rest-assured : 3.1.0. JSON processing: Gson 2.6.1. Only for Java8 
+        rest-assured - HTTP client: rest-assured : 3.1.0. JSON processing: Gson 2.6.1. Only for Java8
 ```
 
 Your config file for Java can look like
@@ -802,7 +802,7 @@ Here are some companies/projects using Swagger Codegen in production. To add you
 - [Hewlett Packard Enterprise](https://hpe.com)
 - [High Technologies Center](http://htc-cs.com)
 - [Hootsuite](https://hootsuite.com/)
-- [Huawei Cloud](http://www.huaweicloud.com/en-us/product/cs.html) 
+- [Huawei Cloud](http://www.huaweicloud.com/en-us/product/cs.html)
 - [Husbanken](https://www.husbanken.no)
 - [IBM](https://www.ibm.com)
 - [IMS Health](http://www.imshealth.com/en/solution-areas/technology-and-applications)
@@ -1109,7 +1109,7 @@ The Swagger Codegen project is intended as a benefit for users of the Swagger /
 * Generated code is intentionally _not_ subject to the parent project license
 
 When code is generated from this project, it shall be considered **AS IS** and owned by the user of the software.  There are no warranties--expressed or implied--for generated code.  You can do what you wish with it, and once generated, the code is your responsibility and subject to the licensing terms that you deem appropriate.
- 
+
 License
 -------
 
@@ -1127,4 +1127,3 @@ limitations under the License.
 
 ---
 <img src="http://swagger.io/wp-content/uploads/2016/02/logo.jpg"/>
-