Add changelog entry

CHANGELOG.md

@@ -22,6 +22,7 @@ Release channels have their own copy of this changelog:
   * Added allow_commission_decrease_at_any_time feature which will allow commission on a vote account to be
     decreased even in the second half of epochs when the commission_updates_only_allowed_in_first_half_of_epoch
     feature would have prevented it
+  * Forbid multiple values for the `--signer` CLI flag, forcing users to specify multiple occurrences of `--signer`, one for each signature
 * Upgrade Notes
   * `solana-program` and `solana-sdk` default to support for Borsh v1, with
 limited backward compatibility for v0.10 and v0.9. Please upgrade to Borsh v1.