Update cisco-duo readme

[cherryleaf]

Added link to Duo documentation

Type of change

• Enhancement

What does this PR do?

Added link to Duo 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: 2022-04-27T21:36:47.753+0000

• Duration: 19 min 17 sec

Test stats 🧪

Test	Results
Failed	0
Passed	54
Skipped	0
Total	54

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (5/5)	💚
Files	100.0% (5/5)	💚 2.857
Classes	100.0% (5/5)	💚 2.857
Methods	100.0% (50/50)	💚 11.905
Lines	91.506% (571/624)	👍 2.669
Conditionals	100.0% (0/0)	💚