Merge branch 'doc/link-coverage-badge-to-coverage-report' into 'master'

Add link to test coverage report to README

## What does this MR do?

Since we have GitLab Pages with test coverage report, it makes sense to link coverage badge there, instead of linking to commits page.

See merge request !6772

README.md

 # GitLab
 
-[![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)
+[![Build status](https://gitlab.com/gitlab-org/gitlab-ce/badges/master/build.svg)](https://gitlab.com/gitlab-org/gitlab-ce/commits/master)
+[![CE coverage report](https://gitlab.com/gitlab-org/gitlab-ce/badges/master/coverage.svg?job=coverage)](http://gitlab-org.gitlab.io/gitlab-ce/coverage-ruby)
 [![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)