Advertise Monocypher's features more clearly

Advertise Monocypher's features more clearly

For the second time, I've stumbled upon a user that believed Monocypher
did not support public key cryptography, despite the presence of a
fairly high-level key exchange API.

Looking at the README and website, I noticed that while key exchange is
fairly well publicised, "public key cryptography" is not.  I suspect
many users simply don't know that key exchange is a valid way to do
public key cryptography.  In hindsihght, we should not expected them to.

Let's not dwell on how many potential users we may have lost to this
oversight.  Hopefully, writing "Public key cryptography" directly will
help people notice.