Update README.md for Metricbeat on K8s

[constanca-m]

What does this PR do?

Updated README.md file for Metricbeat on Kubernetes to explain to the user how to run Metricbeat locally or on cloud.

Why is it important?

The updated file makes clear to the user how to run Metricbeat on Kubernetes.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• Test example on kind and on GKE

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @constanca-m? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2022-10-18T10:35:50.725+0000

• Duration: 8 min 22 sec

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