add LGTM grade

[no ci]

Author: tomato42

README.md

@@ -3,6 +3,8 @@
 [![Build Status](https://travis-ci.com/tlsfuzzer/python-ecdsa.svg?branch=master)](https://travis-ci.com/tlsfuzzer/python-ecdsa)
 [![Coverage Status](https://coveralls.io/repos/github/tlsfuzzer/python-ecdsa/badge.svg?branch=master)](https://coveralls.io/github/tlsfuzzer/python-ecdsa?branch=master)
 [![condition coverage](https://img.shields.io/badge/condition%20coverage-84%25-yellow)](https://travis-ci.com/github/tlsfuzzer/python-ecdsa/jobs/456999547#L586)
+[![Language grade: Python](https://img.shields.io/lgtm/grade/python/g/tlsfuzzer/python-ecdsa.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/tlsfuzzer/python-ecdsa/context:python)
+[![Total alerts](https://img.shields.io/lgtm/alerts/g/tlsfuzzer/python-ecdsa.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/tlsfuzzer/python-ecdsa/alerts/)
 [![Latest Version](https://img.shields.io/pypi/v/ecdsa.svg?style=flat)](https://pypi.python.org/pypi/ecdsa/)
 ![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg?style=flat)