message: docs: add installation to README
diff: @@ -15,6 +15,12 @@ It's written in pure JavaScript:
  - Works with Node.js in Mac/Linux/Windows
  - Works in browsers
  +# How to install
  +
  +```console
  +$ npm install @teppeis/kintone-plugin-packer
  +```
  +
  # Usage
  ```console
  