add logo
add logo

Former-commit-id: c20a8c5b491ae56d5cff6412989ec49e5f747101 [formerly da7f38c9bc724b249130798edc72a27ca8af2c33]
Former-commit-id: 591257b161e49d2cf416fe4809b9e9c1c425571d
LibraryCarpentry/librarycarpentry.github.io,LibraryCarpentry/librarycarpentry.github.io,LibraryCarpentry/librarycarpentry.github.io