Add Inch - quality metrics for better documentation

**As a** developer 
**In order to** raise the inline documentation visibility in a [sound, transparent][inch-ci] and [reasonable][philo] way
**I want to** have quality metrics
**And** I want them to be visible as a `README` badge 

  [philo]: http://trivelop.de/inch
  [inch-ci]: http://inch-ci.org/learn_more