docs: fix bbr notes (backport #4002)

[mergify]

Overview

adds to the release notes to make sure folks know how to enable bbr

---

This is an automatic backport of pull request #4002 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of fae229c has failed:

On branch mergify/bp/v2.x/pr-4002
Your branch is up to date with 'origin/v2.x'.

You are currently cherry-picking commit fae229c7.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   README.md

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/release-notes/release-notes.md

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally