bip138: add acknowledgements

bip-0138.md

@@ -590,4 +590,11 @@ version of this BIP, which is not compatible: its magic is `BEB` rather than `BI
 
 ## Acknowledgements
 
-// TBD
+This BIP builds on the backup scheme proposed by @bigspider on
+[Delving Bitcoin](https://delvingbitcoin.org/t/a-simple-backup-scheme-for-wallet-accounts/1607),
+who also suggested the payload padding design.
+
+Thanks to @Sjors for extensive reviews, many design contributions to the payload
+encoding, and a proof-of-concept implementation on top of Bitcoin Core. Thanks
+also to @murchandamus, @jonatack, @craigraw, @jp1ac4, and @torkelrogstad for
+their reviews and suggestions.