[checkpoint] Add product link to readme

[cherryleaf]

Type of change

• Enhancement

What does this PR do?

Added link to check point documentation

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

Author's Checklist

• [ ]

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[elasticmachine]

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• Start Time: 2022-04-05T23:34:15.332+0000

• Duration: 15 min 21 sec

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Passed	8
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[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[andrewkroh]

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