message: docs: add TOC in README
diff: [![npm version](https://badge.fury.io/js/%40kintone%2Frest-api-client.svg)](https://badge.fury.io/js/%40kintone%2Frest-api-client)
  +- [Installation](#installation)
  +- [Usage](#usage)
  +- [Parameters for `KintoneRestAPIClient`](#parameters-for-kintonerestapiclient)
  +- [Error Handling](#error-handling)
  +- [References](#references)
  +- [Contribution Guide](#contribution-guide)
  +- [License](#license)
  +
  ## Installation
  ### 1. Install with `npm`
  