Add backwards compat section making reference to ln address

Author: TheBlueMatt

bip-XXXX.mediawiki

@@ -101,6 +101,10 @@ Lightning payers should consider utilizing DNS resolution over native onion mess
 
 In most cases where payments are accepted from any third-party, user enumeration is practical by simply attempting to send small value payments to a list of possible user names. However, storing all valid users in the DNS directly may make such enumeration marginally more practical. Thus, those wishing to avoid such enumeration should carefully ensure all DNS names return valid payment instructions. Note when doing so that wildcard records are identified as such by the DNSSEC RRSIG labels counter and are differentiable from non-wildcard records.
 
+== Backwards Compatibility ==
+
+This work is intended to extend and subsume the existing "Lightning Address" scheme, which maps similar names (without the ₿ prefix) using HTTPS servers to Lightning BOLT 11 payment instructions. Wallets implementing this scheme MAY fall back to existing "Lightning Address" logic if DNS resolution fails but SHOULD NOT do so after this scheme is sufficiently broadly deployed to avoid leaking sender IP address information.
+
 == Examples ==
 
 `matt@mattcorallo.com` resolves to