Fix 0.2 CHANGELOG to note that offers will break on downgrade

It turns out we also switched the key we use to authenticate offers
*created* in the 0.2 upgrade and as a result downgrading to 0.2
will break any offers created on 0.2. This wasn't intentional but
it doesn't really seem worth fixing at this point, so just document
it.

Author: TheBlueMatt

CHANGELOG.md

@@ -201,6 +201,8 @@
    generated for inclusion in BOLT 12 `Offer`s will no longer be accepted. As
    most blinded message paths are ephemeral, this should only invalidate issued
    BOLT 12 `Refund`s in practice (#3917).
+ * Blinded message paths included in BOLT 12 `Offer`s generated by LDK 0.2 will
+   not be accepted by prior versions of LDK after downgrade (#3917).
  * Once a channel has been spliced, LDK can no longer be downgraded.
    `UserConfig::reject_inbound_splices` can be set to block inbound ones (#4150)
  * Downgrading after setting `UserConfig::enable_htlc_hold` is not supported