identify: Document identify behavior for asymetrical protocols (#327)

* identify: Document identify behavior for asymetrical protocols

> Note on asymmetrical protocols: Assume an asymmetrical
request-response style protocol `foo` where some clients only support
initiating requests while some servers (only) support responding to
requests. To prevent clients from initiating requests to other clients,
which given them being clients they fail to respond, clients should not
list `foo` in their `protocols` list.

Question emerged in #324.

* identify/README: Reword to inbound substreams