adding LGTM code quality badge

README.md

@@ -2,6 +2,7 @@ srsLTE
 ========
 
 [![Build Status](https://travis-ci.org/srsLTE/srsLTE.svg?branch=master)](https://travis-ci.org/srsLTE/srsLTE)
+[![Language grade: C/C++](https://img.shields.io/lgtm/grade/cpp/g/srsLTE/srsLTE.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/srsLTE/srsLTE/context:cpp)
 
 srsLTE is a free and open-source LTE software suite developed by SRS (www.softwareradiosystems.com). 
 See the srsLTE project pages (www.srslte.com) for documentation, guides and project news.