add back in CC-BY-SA license for the sourmash logo

[ctb]

ref https://github.com/sourmash-bio/sourmash/pull/1127/files#diff-47c5f70f455a0039334dd042f30fbb585c977a73ad838ee4747e5cd304ffb412 - somehow got removed. Maybe figure out why. And, in any case, add it back!