Update libgit2 attributions

Signed-off-by: Paulo Gomes <paulo.gomes@weave.works>

Author: Paulo Gomes

ATTRIBUTIONS.md

@@ -1203,6 +1203,18 @@ STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 OF THE POSSIBILITY OF SUCH DAMAGE.
 
+----------------------------------------------------------------------
+
+The xoroshiro256** implementation is licensed in the public domain:
+
+Written in 2018 by David Blackman and Sebastiano Vigna (vigna@acm.org)
+
+To the extent possible under law, the author has dedicated all copyright
+and related and neighboring rights to this software to the public domain
+worldwide. This software is distributed without any warranty.
+
+See <http://creativecommons.org/publicdomain/zero/1.0/>.
+
 ***
 
 ## zlib