Split shields in multiple lines

Author: Vasileios Karakasis

README.md

@@ -1,12 +1,16 @@
 [![Build Status](https://travis-ci.org/eth-cscs/reframe.svg?branch=master)](https://travis-ci.org/eth-cscs/reframe)
 [![Documentation Status](https://readthedocs.org/projects/reframe-hpc/badge/?version=latest)](https://reframe-hpc.readthedocs.io/en/latest/?badge=latest)
 [![codecov.io](https://codecov.io/gh/eth-cscs/reframe/branch/master/graph/badge.svg)](https://codecov.io/github/eth-cscs/reframe)
+
 ![GitHub release (latest by date including pre-releases)](https://img.shields.io/github/v/release/eth-cscs/reframe?include_prereleases)
-![GitHub contributors](https://img.shields.io/github/contributors-anon/eth-cscs/reframe)
 ![GitHub commits since latest release](https://img.shields.io/github/commits-since/eth-cscs/reframe/latest)
+![GitHub contributors](https://img.shields.io/github/contributors-anon/eth-cscs/reframe)
+
 [![PyPI version](https://badge.fury.io/py/ReFrame-HPC.svg)](https://badge.fury.io/py/ReFrame-HPC)
 ![PyPI - Python Version](https://img.shields.io/pypi/pyversions/reframe-hpc)
+
 [![Slack](https://reframe-slack.herokuapp.com/badge.svg)](https://reframe-slack.herokuapp.com/)
+
 [![License](https://img.shields.io/badge/License-BSD%203--Clause-blue.svg)](https://opensource.org/licenses/BSD-3-Clause)