Add Go Report Card badge

Signed-off-by: Takuya Noguchi <[email protected]>

Author: tnir

README.md

@@ -2,6 +2,7 @@
 
 [![pipeline status](https://gitlab.com/tnir/docs.djangoproject.jp/badges/master/pipeline.svg)](https://gitlab.com/tnir/docs.djangoproject.jp/commits/master)
 [![coverage report](https://gitlab.com/tnir/docs.djangoproject.jp/badges/master/coverage.svg)](https://gitlab.com/tnir/docs.djangoproject.jp/commits/master)
+[![Go Report Card](https://goreportcard.com/badge/gitlab.com/tnir/docs.djangoproject.jp)](https://goreportcard.com/report/gitlab.com/tnir/docs.djangoproject.jp)
 
 Mirror from: https://gitlab.com/tnir/docs.djangoproject.jp