Add silk optional license

https://wiki.debian.org/DFSGLicenses#Creative_Commons_Attribution_Share-Alike_.28CC-BY-SA.29_v3.0

"In contrast to the CC-SA 1.0 license, version 3.0 is considered to be compatible to the DFSG. In addition, the version 2.0 and 2.5 are NOT transitively compatible because of clause 4b, since that only allows redistribution of derivative works under later versions of the license."