add coverage badge (jenkinsci#147)

README.md

@@ -1,6 +1,7 @@
 # Build User Vars Plugin
 
 [![Build Status](https://ci.jenkins.io/job/Plugins/job/build-user-vars-plugin/job/master/badge/icon)](https://ci.jenkins.io/job/Plugins/job/build-user-vars-plugin/job/master/)
+[![Coverage](https://ci.jenkins.io/job/Plugins/job/build-user-vars-plugin/job/master/badge/icon?status=${instructionCoverage}&subject=coverage&color=${colorInstructionCoverage})](https://ci.jenkins.io/job/Plugins/job/build-user-vars-plugin/job/master/coverage)
 [![Contributors](https://img.shields.io/github/contributors/jenkinsci/build-user-vars-plugin.svg)](https://github.com/jenkinsci/build-user-vars-plugin/graphs/contributors)
 [![Jenkins Plugin](https://img.shields.io/jenkins/plugin/v/build-user-vars-plugin.svg)](https://plugins.jenkins.io/build-user-vars-plugin)
 [![GitHub release](https://img.shields.io/github/release/jenkinsci/build-user-vars-plugin.svg?label=changelog)](https://github.com/jenkinsci/build-user-vars-plugin/releases/latest)
@@ -13,7 +14,7 @@ Set of **environment variables** that describe the user who started the build.
 The plugin provides the following environment variables:
 
 | Variable                 | Description                        |
-| ------------------------ | ---------------------------------- |
+|--------------------------|------------------------------------|
 | BUILD\_USER              | Full name (first name + last name) |
 | BUILD\_USER\_FIRST\_NAME | First name                         |
 | BUILD\_USER\_LAST\_NAME  | Last name                          |