update Cyberark Readme

[ellis-elastic]

added link to vendor documentation

• Enhancement

What does this PR do?

added link to vendor 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

• [ ]

How to test this PR locally

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• Start Time: 2022-06-21T19:09:07.727+0000

• Duration: 17 min 3 sec

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Test	Results
Failed	0
Passed	7
Skipped	0
Total	7

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (1/1)	💚
Files	100.0% (1/1)	💚 3.44
Classes	100.0% (1/1)	💚 3.44
Methods	77.778% (7/9)	👎 -11.151
Lines	83.051% (49/59)	👎 -6.9
Conditionals	100.0% (0/0)	💚