docs(readme): add Greenkeeper badge

greenkeeper[bot] · greenkeeper[bot] · commit 99974fd79c43 · 2018-11-23T13:28:22.000Z
diff --git a/README.md b/README.md
@@ -184,4 +184,4 @@ gulp.task('images', () =>
 ```
 
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-[![NPM](https://nodei.co/npm/gulp-srcset.png?downloads=true&downloadRank=true&stars=true)](https://nodei.co/npm/gulp-srcset/)
+[![NPM](https://nodei.co/npm/gulp-srcset.png?downloads=true&downloadRank=true&stars=true)](https://nodei.co/npm/gulp-srcset/) [![Greenkeeper badge](https://badges.greenkeeper.io/TrigenSoftware/gulp-srcset.svg)](https://greenkeeper.io/)
