Added KDC_ERR_CLIENT_NOT_TRUSTED problem to README

[six-two]

Hi, thank you for your useful tool. I recently tried out your tool in a lab and ran into the KDC_ERR_CLIENT_NOT_TRUSTED error. It took me a bunch of time to find out that the CRL flag is needed to bypass this error, at least according to certipy’s README:

If the KDC returns KDC_ERR_CLIENT_NOT_TRUSTED, it means that the forging was not correct. This usually happens because of a missing certificate revocation list (CRL) in the certificate.

To save others some time and frustration, it may be useful to add a hint about this into the README.