added basic Open Graph tags to enable sharing

The basic metadata, described at opg.me is included.

In addition the og:description property is filled with the first
paragraph from the "Why do we need this?" section.

This was tested with two social networks: Facebook, using the
[Facebook Open Graph debugger](https://developers.facebook.com/tools/debug)
and [XING](https://www.xing.com).
Merge pull request #3 from JanAhrens/master

added basic Open Graph tags to enable sharing