Add link to Phoenix implementation

And feedback from @robbiehanson.

Author: t-bast

blip-0042.md

@@ -103,8 +103,8 @@ The reader of `invoice_request`:
       - MUST use the received `contact_secret` whenever paying that contact.
       - MUST use the received `payer_offer` whenever paying that contact.
       - If `payer_bip_353_name` was included:
-        - MUST use it to fetch a `payer_offer` if none was included.
-        - MUST use it to refresh the `payer_offer` when it expires.
+        - SHOULD use it to fetch a `payer_offer` if none was included.
+        - SHOULD use it to refresh the `payer_offer` if it expires.
         - MAY use it to refresh the `payer_offer` periodically.
     - MAY manually associate the received `contact_secret` with an existing
       contact, if the user verified that the payment came from this contact.
@@ -258,3 +258,4 @@ section.
 ## Reference Implementations
 
 - lightning-kmp: <https://github.com/ACINQ/lightning-kmp/pull/719>
+- phoenix wallet: <https://github.com/ACINQ/phoenix/pull/645>