Add docs badge to README

Hi there,

I saw that you (somebody else) tried adding this project to http://inch-ci.org yesterday. Unfortunately, the GitHub API had some outages and there were some hiccups during the creation of your project on Inch CI.

This patch adds the Inch CI badge to your repo. You can refresh the badge manually on your [project page](http://inch-ci.org/github/sc21/zbis/branch/master) or you can [configure a webhook to rebuild](http://inch-ci.org/howto/webhook) your badge auto-magically :sparkles: 

If you have any questions or suggestions, just post them here!