signer API: document supported keytypes

[jku]

This relates to #466.

The signer and key support is now modular so it's not as clear what key types and signers are supported -- and we don't want to point people to keys module like the signer API docstrings currently do.

We should collect into one place

• What keys and signers are supported
• How to sign with signer API
• How to initialize keys (import/generate) with signer API
• how to create and use custom keys and signers

Once #466 is done, I think we can do this by rewriting the README using the signer API.