Add description of expansion header X11 (#80)

This adds a description of the most important parts of the
expansion header X11.

Signed-off-by: Stefan Wahren <stefan.wahren@chargebyte.com>
Signed-off-by: Michael Heimpold <michael.heimpold@chargebyte.com>
Co-authored-by: Michael Heimpold <michael.heimpold@chargebyte.com>

Author: lategoodbye