Add some release notes for 0.28.4

Summary: As per title.

Test Plan: Read it.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14873

doc/release-notes.md

@@ -8,3 +8,6 @@ This release includes the following features and fixes:
  - The avalanche peers are now dumped to a file upon node shutdown and reloaded
  upon startup to allow for faster avalanche bootstraping. This feature can be
  disabled by using the `-persistavapeers=0` option.
+ - `bitcoin-seeder` was not working properly since version 0.28.2 and has been
+ fixed. While crawling for peers appropriately, it would have fail to mark any
+ peer as good due to a bug.