update Fortinet readme

[ellis-elastic]

added link to developer documentation

Type of change

• Enhancement

What does this PR do?

added link to developer 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

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• Start Time: 2022-06-13T14:35:31.237+0000

• Duration: 25 min 11 sec

Test stats 🧪

Test	Results
Failed	0
Passed	30
Skipped	0
Total	30

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (4/4)	💚
Files	100.0% (7/7)	💚 3.432
Classes	100.0% (7/7)	💚 3.432
Methods	94.203% (65/69)	👍 5.314
Lines	91.056% (1242/1364)	👍 1.163
Conditionals	100.0% (0/0)	💚

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