[JavaScript][ES6] Incorrect content in the generated README.md

Description

1. When generating the SDK with configuration file js-config.json with useES6 set to true:

   {
       "useES6" : true
   }
   

   The README.md in the generated package contains code examples that is still in ES5. i.e.

   var coolAPI = require('cool_api');
   
   var api = new coolApi.ModuleFooApi()
   ...
   

   which should be:

   import coolAPI from 'cool_api';
   
   let apiInstance = new coolApi.ModuleFooApi();
   ...
   

2. Also the Local development section that used to be in the ES5 SDK is also missing from the ES6 SDK. Perhaps the npm link instructions would still be useful?

Swagger-codegen version

Command line used for generation

java -jar swagger-codegen-cli-2.3.1.jar generate -i api-spec.json -l javascript -o ./js-sdk -c js-config.json

Other Observations

This issue seems only have impacted the README.md that exist at the root folder of the generated package. All other *.md files under /docs folder were generated with the correct example in ES6 syntax.

Reproduce the JavaScript SDK with the shown swagger-codegen command and js-config.json using useES6=true. Compare the generated root README.md with the correctly formatted markdown under /docs; done means the README uses ES6 examples and restores the Local development/npm link guidance.