bip340: Add changelog

bip-0340.mediawiki

@@ -287,6 +287,12 @@ Blind Schnorr signatures could for example be used in [https://github.com/Elemen
 For development and testing purposes, we provide a [[bip-0340/test-vectors.csv|collection of test vectors in CSV format]] and a naive, highly inefficient, and non-constant time [[bip-0340/reference.py|pure Python 3.7 reference implementation of the signing and verification algorithm]].
 The reference implementation is for demonstration purposes only and not to be used in production environments.
 
+== Changelog ==
+
+To help implementors understand updates to this BIP, we keep a list of substantial changes.
+
+* 2022-12: Allow messages of arbitrary size
+
 == Footnotes ==
 
 <references />