commonSourceDirectory config is buggy and wrongly documented

## Bug
The docs say that you can provide `commonSourceDirectory` configuration like `"commonSourceDirectory": "source/common"`, however if you look at the code in the node module you can clearly see that this would not work as expected:

```js
// watch common code
let commonSourceDir = configurator.electronWebpackConfiguration.commonSourceDirectory;
if (commonSourceDir == null) {
// not src/common, because it is convenient to just put some code into src to use it
commonSourceDir = _path.join(configurator.projectDir, "src");
}
```

The code expects either a full custom path to a `commonSourceDirectory` (which is hard to provide via .json config files) or it (correctly) builds the path relatively from the `projectDir` if nothing is specified. The correct code should look like this:

```js
let commonSourceDir = configurator.electronWebpackConfiguration.commonSourceDirectory || 'src';
commonSourceDir = _path.join(configurator.projectDir, commonSourceDir);
```

## Wrongly documented

Additionally the docs suggest that the `commonSourceDirectory` is for shared code … however it actually means the root directory (e.g: `src`/ `source`) where all code lives in, because the file watching relies on that.

… i ran into all of these issue just because i wanted to "customize" the `src` directory to `source` 😉

Start by locating the common-source watching code around configurator.electronWebpackConfiguration.commonSourceDirectory and compare it with the documented JSON configuration. Verify the relative-path behavior using a custom source directory, then update the documentation so commonSourceDirectory is described as the project’s code root rather than only shared code.

Written by the indexing model from the issue text.