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

doc/OS-X-Codesigning.txt

@@ -0,0 +1,53 @@
+Request, load and use OS X codesigning certificates
+---------------------------------------------------
+
+This is a short step-by-step on how to generate a key in the Neo,
+create a certificate request, submit that request to apple, load the
+certificate in the Neo and use it for codesigning.
+
+Prerequisites
+-------------
+
+* a YubiKey Neo with the PIV applet loaded
+* the yubico-piv-tool software
+* the OpenSC software
+* membership in the mac developer program
+
+Steps
+-----
+
+1. Generate a key in slot 9a:
+
+  $ yubico-piv-tool -s 9a -a generate -o public.pem
+
+2. Create a certificate request for app distribution:
+
+  $ yubico-piv-tool -a verify-pin -P 123456 -s 9a -a request-certificate \
+	-S "/CN=Application/" -i public.pem -o application.csr
+
+3. Generate a key in slot 9c:
+
+   $ yubico-piv-tool -s 9c -a generate -o public.pem
+
+4. Create a certificate request for installer distribution:
+
+  $ yubico-piv-tool -a verify-pin -P 123456 -s 9c -a request-certificate \
+	-S "/CN=Installer/" -i public.pem -o installer.csr
+
+5. Go to the Apple developer program page and submit the requests.
+
+6. When the certificates are ready and approved, download them.
+
+7. Load the certificates:
+
+   $ yubico-piv-tool -a import-certificate -s 9a -K DER -i mac_app.cer
+   $ yubico-piv-tool -a import-certificate -s 9c -K DER -i mac_installer.cer
+
+8. Set a new chuid in the applet to make sure nothing is cached for the key:
+
+   yubico-piv-tool -a set-chuid
+
+9. Re-plug the Neo and make sure the certificates show up under the keychain
+"PIV_II" in Keychain Access.
+
+10. Use the certificates as usual with codesign/pkgbuild/productbuild/productsign