message: docs: add short description of rest-api-client on README
diff: [![npm version](https://badge.fury.io/js/%40kintone%2Frest-api-client.svg)](https://badge.fury.io/js/%40kintone%2Frest-api-client)
  +API client for Kintone REST API.
  +It supports both browser environment (Kintone customization & plugin) and Node.js environment.
  +
  - [Installation](#installation)
  - [Browsers support](#browsers-support)
  - [Usage](#usage)
  