Add LGTM grade badge to README

README.md

@@ -5,7 +5,8 @@
 [![GitHub Workflow Status](https://img.shields.io/github/workflow/status/bblanchon/ArduinoJson/Continuous%20Integration?logo=github)](https://github.com/bblanchon/ArduinoJson/actions?query=workflow%3A%22Continuous+Integration%22+branch%3A6.x)
 [![Continuous Integration](https://ci.appveyor.com/api/projects/status/m7s53wav1l0abssg/branch/6.x?svg=true)](https://ci.appveyor.com/project/bblanchon/arduinojson/branch/6.x)
 [![Fuzzing Status](https://oss-fuzz-build-logs.storage.googleapis.com/badges/arduinojson.svg)](https://bugs.chromium.org/p/oss-fuzz/issues/list?sort=-opened&can=1&q=proj:arduinojson)
-[![Coverage Status](https://coveralls.io/repos/github/bblanchon/ArduinoJson/badge.svg?branch=6.x)](https://coveralls.io/github/bblanchon/ArduinoJson?branch=6.x)  
+[![LGTM Grade](https://img.shields.io/lgtm/grade/cpp/github/bblanchon/ArduinoJson?label=quality&logo=lgtm)](https://lgtm.com/projects/g/bblanchon/ArduinoJson/)
+[![Coveralls branch](https://img.shields.io/coveralls/github/bblanchon/ArduinoJson/6.x?logo=coveralls)](https://coveralls.io/github/bblanchon/ArduinoJson?branch=6.x)     
 [![Discord](https://img.shields.io/discord/844900310409150494.svg?logo=discord&logoColor=white&color=007ec6)](https://discord.gg/DzN6hHHD4h)
 [![GitHub stars](https://img.shields.io/github/stars/bblanchon/ArduinoJson?style=flat&logo=github)](https://github.com/bblanchon/ArduinoJson/stargazers)
 [![GitHub Sponsors](https://img.shields.io/github/sponsors/bblanchon?logo=github)](https://github.com/sponsors/bblanchon)