Add CII badge to README [ci skip]

Former-commit-id: 34ea6802

README.md

@@ -3,6 +3,7 @@
 [![build status](https://gitlab.com/gitlab-org/gitlab-ce/badges/master/build.svg)](https://gitlab.com/gitlab-org/gitlab-ce/commits/master)
 [![coverage report](https://gitlab.com/gitlab-org/gitlab-ce/badges/master/coverage.svg?job=coverage)](https://gitlab.com/gitlab-org/gitlab-ce/commits/master)
 [![Code Climate](https://codeclimate.com/github/gitlabhq/gitlabhq.svg)](https://codeclimate.com/github/gitlabhq/gitlabhq)
+![Core Infrastructure Initiative Best Practices](https://bestpractices.coreinfrastructure.org/projects/42/badge)](https://bestpractices.coreinfrastructure.org/projects/42)
 ## Canonical source