Add Social Preview image for Github

[dkastl]

In this repository settings there is a section to configure the "Social Preview image" for Github:

Social preview
Upload an image to customize your repository’s social media preview.

Images should be at least 640×320px (1280×640px for best display).

https://github.com/consento-org/mobile/settings

See also the following template:

[martinheidegger]

Added an illustration: https://opengraphcheck.com/result.php?url=https%3A%2F%2Fgithub.com%2Fconsento-org%2Fmobile#.XlYDYBMzZ24