Add documentation about not accepting PRs for new adapters

[jagthedrummer]

Summary

This just adds some documentation that the Rails team is not accepting PRs for new ActiveJob adapters. This will prevent future potential contributors from spending time on PRs that will never have a chance of being accepted.

Other Information

Inspired by:

• Faktory adapter #32285
• Add SQS for active job #21406 (comment)
• Shoryuken adapter? #23311 (comment)

[rails-bot]

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[rafaelfranca]

We may in future add an adapter for Faktory by default given it get the same traction as Sidekiq, but for now I agree it is better to keep it in the Faktory gem itself.