Improve docs: add Upgrade-Notes; add online support refs

Upgrade-Notes includes v2 to v3 upgrade changes that users should be
aware of.

The updated top-level README helps point users to online support
locations.

Signed-off-by: Josh Cepek <josh.cepek@usa.net>
Add support to change private key passphrases

Adds the 'set-rsa-pass' and 'set-ec-pass' commands. Both are able to set
or remove a passphrase, and can optionally act on a direct file path
instead of a relative filename base name.

Signed-off-by: Josh Cepek <josh.cepek@usa.net>