PSK format unclear when using `Config` programatically

libp2p PSK's need to be formatted like that:

```rust
format!("/key/swarm/psk/1.0.0/\n/base16/\n{}", pre_shared_secret)
    .parse()
    .expect("Invalid Pre-Shared Key"),
```

.. this is not communicated clearly to users of the API, so we either 1) inform them that this is the required format or 2) offer an interface which does it automatically