added basic Open Graph tags to enable sharing

The basic metadata, described at ogp.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).

The preview image is not ideal and Facebook is know to display a part of the image.
Maybe someone else can come up with a better version ([without using the Ubuntu logo](https://micahflee.com/2013/11/canonical-shouldnt-abuse-trademark-law-to-silence-critics-of-its-privacy-decisions/)).