Update cisco readme

[cherryleaf]

Added link to Cisco's documentation

Type of change

• Enhancement

What does this PR do?

Added link to Cisco's 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

Related issues

Screenshots

[elasticmachine]

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• Start Time: 2022-05-18T13:27:55.431+0000

• Duration: 24 min 53 sec

Test stats 🧪

Test	Results
Failed	0
Passed	60
Skipped	0
Total	60

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

[efd6]

Note that this package is deprecated, so it's probably not worthwhile adding this here.

[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (4/4)	💚
Files	100.0% (4/4)	💚 3.589
Classes	100.0% (4/4)	💚 3.589
Methods	94.828% (55/58)	👍 6.539
Lines	65.226% (2279/3494)	👎 -23.791
Conditionals	100.0% (0/0)	💚