Add explanation on how this tool relates to YubiKey Manager CLI (i.e. ykman)

[tjanez]

It would be helpful if the README would contain some explanation on how this project (i.e. yubico-piv-tool) relates to the YubiKey Manager CLI (i.e. ykman) and its ykman piv subcommand.

Are there any functional differences between the two?

From a quick search, there appear to be some. For example:

• Add support for management key derivation from PIN like YubiKey Manager #500
• Provide way to export public key for a given slot akin to ykman piv keys export #501

Which tool should users use in which context?