Update README to make it easier to get started.
Update verifyUserIdSelfSig to use new interface.

The pubkey.PublicKey.VerifyUserIdSignature and VerifyUserIdSignatureV3
methods seem to have changed to require a key. Since the method where these
calls exist is called verifyUserIdSelfSig it seemed appropriate to pass the
key we're calling the method on.

After this change, things will at least compile.