Talk about the Silk icon set

The license of the Silk icon set requires a link back to its source.

Signed-off-by: Sebastian Ramacher <sramacher@debian.org>
Explicitly talk about Silk's license

Signed-off-by: Sebastian Ramacher <sramacher@debian.org>
Merge pull request #7 from sebastinas/doc-silk

Add a note about Silk's license