docs(README): add some metadata badges

Including license, Python version, package version, build status and coverange.

CodeKitchen: https://codekitchen.community/t/topic/1134/3

README.md

@@ -1,5 +1,11 @@
 # Flask-CKEditor
 
+![GitHub License](https://img.shields.io/github/license/helloflask/flask-ckeditor%20)
+![PyPI - Python Version](https://img.shields.io/pypi/pyversions/flask-ckeditor)
+![PyPI - Version](https://img.shields.io/pypi/v/flask-ckeditor)
+![Build Status](https://github.com/helloflask/flask-ckeditor/workflows/build/badge.svg)
+[![codecov](https://codecov.io/gh/helloflask/flask-ckeditor/graph/badge.svg?token=bCkkuXW4dX)](https://codecov.io/gh/helloflask/flask-ckeditor)
+
 CKEditor integration for Flask, including image upload, code syntax highlighting, and more.