docs: add permit functionality to allow single transactions registers

Author: jm-clius

FURPS/core/rln_smart_contract.md

@@ -13,6 +13,7 @@
 2. User does not need to wait for merkle tree synchronization and building to start relaying
    or sending messages.
 3. Application does not need to do a Web3 RPC call for every tree change to generate or validate messages.
+4. Application can transfer tokens and register membership with a single transaction.
 
 ## Reliability
 

draft-roadmap/integrate_rln_with_waku_api.md

@@ -132,14 +132,15 @@ See deliverables.
 - [ ] Dogfood: link to dogfooding session/artefact
 - [ ] Docs: links to README.md or docs.waku.org (TBD)
 
-### Improve RLN UX by reducing Web3 RPC calls
+### [Improve RLN UX by reducing contract interactions](https://github.com/waku-org/pm/issues/344)
 
 **Owner**: core research
 
 **Feature**: [RLN Smart Contract](/FURPS/core/rln_smart_contract.md)
 
 **FURPS**:
 - U3. Application does not need to do a Web3 RPC call for every tree change to generate or validate messages.
+- U4. Application can transfer tokens and register membership with a single transaction.
 
 **Checklist**:
 - [ ] Specs: link to specs and/or API definition