Add coverage badge to README

Author: bact

README.md

@@ -8,6 +8,7 @@
 [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.3519354.svg)](https://doi.org/10.5281/zenodo.3519354)
 [![Project Status: Active](https://www.repostatus.org/badges/latest/active.svg)](https://www.repostatus.org/#active)
 [![Codacy Grade](https://app.codacy.com/project/badge/Grade/5821a0de122041c79999bbb280230ffb)](https://www.codacy.com/gh/PyThaiNLP/pythainlp/dashboard?utm_source=github.com&utm_medium=referral&utm_content=PyThaiNLP/pythainlp&utm_campaign=Badge_Grade)
+[![Coverage Status](https://coveralls.io/repos/github/PyThaiNLP/pythainlp/badge.svg?branch=dev)](https://coveralls.io/github/PyThaiNLP/pythainlp?branch=dev)
 [![Google Colab Badge](https://badgen.net/badge/Launch%20Quick%20Start%20Guide/on%20Google%20Colab/blue?icon=terminal)](https://colab.research.google.com/github/PyThaiNLP/tutorials/blob/master/source/notebooks/pythainlp_get_started.ipynb)
 [![Facebook](https://img.shields.io/badge/Facebook-0866FF?style=flat&logo=facebook&logoColor=white)](https://www.facebook.com/pythainlp/)
 [![Chat on Matrix](https://matrix.to/img/matrix-badge.svg)](https://matrix.to/#/#thainlp:matrix.org)