Adding a point about the coding style.

The idea of this pull request is also to agree on the exact coding style we want.
Yes of course. But that is not what you are saying in the doc.

Also you should encourage the same tools as our CI server uses. Because that will throw same errors as our CI.

In this case I think is best idea to encourage what we already have in the README.

```
 ./vendor/bin/phpcs --standard=psr2 ./src
 ./vendor/bin/phpmd src/ text codesize,controversial,design,naming,unusedcode
```