added new mac icon to assets-attribution.txt

license: MIT
Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>

doc/assets-attribution.txt

@@ -55,3 +55,7 @@ Icon: src/qt/res/icons/debugwindow.png
 Designer: Vignoni David
 Site: http://www.oxygen-icons.org/
 License: Oxygen icon theme is dual licensed. You may copy it under the Creative Common Attribution-ShareAlike 3.0 License or the GNU Library General Public License.
+
+Icon: src/qt/res/icons/bitcoin.icns, src/qt/res/icons/bitcoin.psd
+Designer: Jonas Schnelli
+License: MIT