add inline code documentation quality badge (inch-ci.org)

a badge indicating the quality of inline code documentation based on [inch](http://trivelop.de/inch/) and [inch ci](http://inch-ci.org/).

after merging we most likely want to configure a webhook for https://github.com/opf/openproject. this is described here http://inch-ci.org/howto/webhook.

and then we should improve out docs!