add some text about how to use for codesigning on OS X
add some text about how to use for codesigning on OS X

hirden/yubico-piv-tool,Yubico/yubico-piv-tool,akgood/yubico-piv-tool,akgood/yubico-piv-tool,Yubico/yubico-piv-tool,ato/yubico-piv-tool,hirden/yubico-piv-tool,akgood/yubico-piv-tool,akgood/yubico-piv-tool,Yubico/yubico-piv-tool,ato/yubico-piv-tool