Document which conventions belong in this repo

[lmolkova]

Document that non-OTel conventions (e.g. Kafka client JMX metrics or RabbitMQ Collector Receiver) should not be contributed.

This is long-overdue (open-telemetry/opentelemetry.io#4063, #338)

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.