Add logo for the project

[scaronni]

Discussed in #557

Unfortunately we're name matching dkms.de (Deutsche Knochenmarkspenderdatei - German Bone Marrow Donor Registry), which has a logo with just the letters, as I thought I would like.

[scaronni]

Nobody proposed anything, so I just came up with this. Nothing too fancy:

• The dark container represents the running kernel.
• The green blocks represent DKMS replacement or added modules.
• The red block represents a module overridden by DKMS.

[scaronni]

It sucks? Maybe. Nobody proposed anything...