Update README.md

[atihkin]

What does this PR do?

This update to the readme attempts to clarify that this integration is only possible when self-hosting Temporal.

Motivation

Users are confused about whether this Datadog + Temporal integration works for Temporal Cloud or not.

Additional Notes

Feel free to modify the language. Also context if it helps, I'm a Product Manager at Temporal.

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] Changelog entries must be created for modifications to shipped code
• [ ] Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• [ ] If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged