Add some release notes for 0.28.1

Summary: As per title.

Test Plan: Read it.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14688

doc/release-notes.md

@@ -7,3 +7,7 @@ Bitcoin ABC version 0.28.1 is now available from:
 This release includes the following features and fixes:
  - New `getstakingreward` and `setstakingreward` RPCs have been added to
  retrieve and manually set the staking reward payout for a given block.
+ - The `getremoteproofs` RPC has been added to get the proof set gathered from
+ other avalanche peers.
+ - The avalanche proofs retrieval has been enhanced to make the node more
+ resilient to peer disconnects.