rendezvous/: Remove PoW from the spec (#342)

Relates to https://github.com/libp2p/specs/pull/334.
Fixes https://github.com/libp2p/specs/issues/337.

rendezvous/README.md

@@ -30,7 +30,7 @@ and spec status.
 - [The Protocol](#the-protocol)
    - [Registration Lifetime](#registration-lifetime)
    - [Interaction](#interaction)
-   - [Proof of Work](#proof-of-work)
+   - [Spam mitigation](#spam-mitigation)
    - [Protobuf](#protobuf)
 - [Recommendations for Rendezvous Points configurations](#recommendations-for-rendezvous-points-configurations)
 
@@ -178,15 +178,15 @@ R -> D: {[REGISTER{my-app, {QmE, AddrE}}],
          c3}
 ```
 
-### Proof of Work
+### Spam mitigation
 
 The protocol as described so far is susceptible to spam attacks from
 adversarial actors who generate a large number of peer identities and
-register under a namespace of interest (eg: the relay namespace). This
-can be mitigated by requiring a Proof of Work scheme for client
-registrations.
+register under a namespace of interest (eg: the relay namespace).
 
-This is TBD before finalizing the spec.
+It is TBD how exactly such attacks will be mitigated.
+See https://github.com/libp2p/specs/issues/341 for a discussion on this
+topic.
 
 ### Protobuf