update wording related to config.json

Author: wing328

README.md

@@ -496,8 +496,15 @@ java -jar modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate \
   -o samples/client/petstore/java \
   -c path/to/config.json
 ```
-Supported config options can be different per language. Running `config-help -l {lang}` will show available options.  **These options are applied
-by passing them with `-D{optionName}={optionValue}**.
+and `config.json` contains the following as an example:
+```
+{
+  "apiPackage" : "petstore"
+}
+```
+
+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)
 
 ```
 java -jar modules/swagger-codegen-cli/target/swagger-codegen-cli.jar config-help -l java