Add OpenHAB custom service

[andreapier]

Description

I created custom service for OpenHAB.
I also updated the relevant documentation for custom services. This is a screenshot using data from my local deploy:

Fixes # N/A

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [X] I've read & comply with the contributing guidelines
• [X] I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• [X] I have made corresponding changes to the documentation (README.md).
• [X] I've checked my modifications for any breaking changes, especially in the config.yml file

[netlify[bot]]

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