Add a sonar tutorial

It would be great to help people to setup nyc with sonar.

Here is my setup:

```
npm install xunit-file --save-dev
```

add the following to the mocha command:

```
--reporter xunit-file --reporter-options file='./report/test-all.xml'
```

---

This is what I use (typescript + webpack + react) :

```
nyc mocha-webpack --webpack-config webpack.coverage.config.js \"(tests|web_modules)/**/*.test.\\?(ts|tsx)\" --require source-map-support/register --reporter mocha-sonar-reporter --reporter-options file='./coverage/TEST-xunit.xml'
```

webpack config:

```

// Enable sourcemaps for debugging webpack's output.
devtool: "cheap-module-source-map",
module: {
loaders: [
{
test: /\.tsx{0,1}?$/,
exclude: [/node_modules/, /json/],
loaders: ['istanbul-instrumenter', "babel-loader", "ts-loader"]
}
]
}

plugins: [
// Allow to define React as a global variable for JSX/TSX.
new webpack.ProvidePlugin({"React": "react",}),
]
```