message: docs: add plugin-manifest-validator in root README
diff: @@ -11,6 +11,7 @@ This repository is a monorepo for Kintone development tools!
  | ---------------------------------------------------------- | :-----------------------------------: | :--------------------------------------------------------------------------------------------------------------------------------------: |
  | [@kintone/rest-api-client](packages/rest-api-client) | An API client for Kintone REST API | [![npm version](https://badge.fury.io/js/%40kintone%2Frest-api-client.svg)](https://badge.fury.io/js/%40kintone%2Frest-api-client) |
  | [@kintone/customize-uploader](packages/customize-uploader) | An uploader for Kintone customization | [![npm version](https://badge.fury.io/js/%40kintone%2Fcustomize-uploader.svg)](https://badge.fury.io/js/%40kintone%2Fcustomize-uploader) |
  +| [@kintone/plugin-manifest-validator](packages/plugin-manifest-validator) | A validator for `manifest.json` of Kintone plugin | [![npm version](https://badge.fury.io/js/%40kintone%2Fplugin-manifest-validator.svg)](https://badge.fury.io/js/%40kintone%2Fplugin-manifest-validator) |
  ## Contribution Guide
  