README.md: add LGTM.com code quality grade for C/C++

Closes curl#2857

Author: sj

README.md

@@ -4,7 +4,9 @@
 [![Coverity passed](https://scan.coverity.com/projects/curl/badge.svg)](https://scan.coverity.com/projects/curl)
 [![Build Status](https://travis-ci.org/curl/curl.svg?branch=master)](https://travis-ci.org/curl/curl)
 [![Coverage Status](https://coveralls.io/repos/github/curl/curl/badge.svg)](https://coveralls.io/github/curl/curl)
-[![Backers on Open Collective](https://opencollective.com/curl/backers/badge.svg)](#backers) [![Sponsors on Open Collective](https://opencollective.com/curl/sponsors/badge.svg)](#sponsors)
+[![Backers on Open Collective](https://opencollective.com/curl/backers/badge.svg)](#backers)
+[![Sponsors on Open Collective](https://opencollective.com/curl/sponsors/badge.svg)](#sponsors)
+[![Language Grade: C/C++](https://img.shields.io/lgtm/grade/cpp/g/curl/curl.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/curl/curl/context:cpp)
 
 Curl is a command-line tool for transferring data specified with URL
 syntax. Find out how to use curl by reading [the curl.1 man