message: add yarn lock info to contributing.md
diff: @@ -259,6 +259,10 @@ superset runserver -d -p 8081
  npm run dev
  ```
  +#### Upgrading npm packages
  +
  +Should you add or upgrade a npm package, which involves changing `package.json`, you'll need to re-run `yarn install` and push the newly generated `yarn.lock` file so we get the reproducible build. More information at (https://yarnpkg.com/blog/2016/11/24/lockfiles-for-all/)
  +
  ## Testing
  Before running python unit tests, please setup local testing environment:
  