docs: Add types friendly badge 📚 (dowjones#229)

## What does it do?

Adds a "types (friendly) badge" to Readme

## Type of change

Please delete options that are not relevant.

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
- [X] This change requires a documentation update

README.md

@@ -4,7 +4,7 @@
 
 [![NPM version][npm-image]][npm-url] [![gzip][gzip-image]][gzip-url] [![npm download][download-image]][npm-url]
 
-[![build status][travis-image]][travis-url] [![Test coverage][coveralls-image]][coveralls-url] [![Commitizen friendly][commitizen]][commitizen-url] [![semantic-release][semantic-release]][semantic-release-url] [![All Contributors](https://img.shields.io/badge/all_contributors-26-orange.svg?style=flat-square)](#contributors)
+[![build status][travis-image]][travis-url] [![Test coverage][coveralls-image]][coveralls-url] [![Commitizen friendly][commitizen]][commitizen-url] [![semantic-release][semantic-release]][semantic-release-url] [![All Contributors][all-contributors-url]](#contributors) ![npm type definitions][types-url]
 
 [npm-image]: http://img.shields.io/npm/v/react-dropdown-tree-select.svg?style=flat-square
 [npm-url]: http://npmjs.org/package/react-dropdown-tree-select
@@ -19,6 +19,8 @@
 [gzip-url]: https://unpkg.com/react-dropdown-tree-select/dist/react-dropdown-tree-select.js
 [semantic-release]: https://img.shields.io/badge/%20%20%F0%9F%93%A6%F0%9F%9A%80-semantic--release-e10079.svg?style=flat-square
 [semantic-release-url]: https://github.com/semantic-release/semantic-release
+[all-contributors-url]: https://img.shields.io/badge/all_contributors-26-orange.svg?style=flat-square
+[types-url]: https://img.shields.io/npm/types/react-dropdown-tree-select.svg?style=flat-square
 
 ## React Dropdown Tree Select