doc: Add release notes for 32521 (MAX_TX_LEGACY_SIGOPS)

Author: MarcoFalke

doc/release-notes-32521.md

@@ -0,0 +1,9 @@
+### Policy
+
+The maximum number of potentially executed legacy signature operations in a
+single standard transaction is now limited to 2500. Signature operations in all
+previous output scripts, in all input scripts, as well as all P2SH redeem
+scripts (if there are any) are counted toward the limit. The new limit is
+assumed to not affect any known typically formed standard transactions. The
+change was done to prepare for a possible BIP54 deployment in the future.
+(#32521)