message: Add Sonar badges
diff: How to compile RINGMesh {#ringmesh_compiling}
  =======================
  -[![Build status](https://ci.appveyor.com/api/projects/status/nlso0s96wcuge2vn/branch/master?svg=true)](https://ci.appveyor.com/project/ringmesh/ringmesh/branch/master)
  -[![Build Status](https://travis-ci.org/ringmesh/RINGMesh.svg?branch=master)](https://travis-ci.org/ringmesh/RINGMesh)
  -[![Coverage Status](https://coveralls.io/repos/bitbucket/ring_team/ringmeshcoverage/badge.svg?branch=default)](https://coveralls.io/bitbucket/ring_team/ringmeshcoverage?branch=default)
  +[![Travis build](https://ci.appveyor.com/api/projects/status/nlso0s96wcuge2vn/branch/master?svg=true)](https://ci.appveyor.com/project/ringmesh/ringmesh/branch/master)
  +[![Appveyor build](https://travis-ci.org/ringmesh/RINGMesh.svg?branch=master)](https://travis-ci.org/ringmesh/RINGMesh)
  +[![Sonar quality](https://sonarcloud.io/api/badges/gate?key=ringmesh)](https://sonarcloud.io/dashboard/index/ringmesh)
  +[![Coverage](https://sonarcloud.io/api/badges/measure?key=ringmesh&metric=coverage)](https://sonarcloud.io/dashboard/index/ringmesh)
  RINGMesh is tested under Linux (64 bits) and Windows (64 bits).
  You will need CMake (version >= 3.1). There is no other dependency (everything
  