Merge branch 'docs/add-tests-coverage-badge-to-readme' into 'master'

Add tests coverage report badge to README

## What does this MR do?

This MR adds tests coverage badge to project's README.

The job it reads coverage value from is `coverage`, and it is configured in out CI test suite to calculate compound coverage from all test jobs running in parallel.

This will pick the latest successful pipeline to get `coverage` job and coverage value.

For now, this will report test coverage only for backend test suite (Ruby).

See merge request !5925

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)
 [![Code Climate](https://codeclimate.com/github/gitlabhq/gitlabhq.svg)](https://codeclimate.com/github/gitlabhq/gitlabhq)
 
 ## Canonical source