docs: add the dev@ mailing list to the README and CONTRIBUTING

[justinmclean]

Which issue does this PR address?

N/A

Rationale

The dev@ list is where project decisions are made, so newcomers should be able to find it.

What changed?

Both pointed only to Discord and GitHub Discussions; added dev list.

Local Execution

• Passed

AI Usage

AI was use reviewed by a human.

[github-actions]

Thanks for the PR. It is labeled S-waiting-on-review and queued for review.

Slash commands (own line, regular comment) move it around the queue:

• /ready - back to S-waiting-on-review after addressing feedback
• /author - flip to S-waiting-on-author while you finish changes
• /request-review @user-or-team - request a reviewer
• /pin - exempt the PR from the stale bot, /unpin to undo

See CONTRIBUTING.md for details.