change defaults and add info to #Require touch
change defaults and add info to #Require touch

As mentioned in #197, the previous behaviour would require users to
touch their key any time an authentication, signing, or encryption
operation was performed. In some situations, this behaviour would be
undesirable and the only way to revert it would be fully resetting the
key and starting from scratch. Rather than using `fixed`, this commit
simply turns the feature `on` so the user can change it later if they
wish.

Additionally, a note about the other policies was included so users can
decide for themselves which fits their situation better.

drduh/YubiKey-Guide