Add steps for renewing (not rotating) sub-keys
Add steps for renewing (not rotating) sub-keys

As discussed in issue #164, the current section on Rotating Keys
presents two alternatives: replacing the existing keys with a newly
generated key or extending the validity of existing keys by changing
their expiration.  However, it only provides instructions for the
first approach.  This commit adds instructions for renewing sub-keys.

I am far from an expert, and am submitting this change mostly in hopes
that it will provide documentation for the next time I need to renew
my sub-keys.  I would welcome any changes or clarifications others
would care to offer.

drduh/YubiKey-Guide