Add grade shield (apache#8344)

README.md

@@ -19,6 +19,7 @@
 
 [![Slack](https://img.shields.io/badge/slack-%23druid-72eff8?logo=slack)](https://druid.apache.org/community/join-slack)
 [![Build Status](https://travis-ci.org/apache/incubator-druid.svg?branch=master)](https://travis-ci.org/apache/incubator-druid)
+[![Language grade: Java](https://img.shields.io/lgtm/grade/java/g/apache/incubator-druid.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/apache/incubator-druid/context:java)
 [![Coverage Status](https://coveralls.io/repos/github/apache/incubator-druid/badge.svg?branch=master)](https://coveralls.io/github/apache/incubator-druid?branch=master)
 <!--- Following badges are disabled until they can be fixed: -->
 <!--- [![Inspections Status](https://img.shields.io/teamcity/http/teamcity.jetbrains.com/s/OpenSourceProjects_Druid_Inspections.svg?label=TeamCity%20inspections)](https://teamcity.jetbrains.com/viewType.html?buildTypeId=OpenSourceProjects_Druid_Inspections) -->