Add note on Apple App Transport Security for the macOS desktop client

[claucambra]

☑️ Resolves

Makes certificate requirements clear for macOS users using the desktop client

[claucambra]

@mgallien ping?

[welcome]

Thanks for your first pull request and welcome to the community! Feel free to keep them coming! If you are looking for issues to tackle then have a look at this selection: https://github.com/nextcloud/documentation/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22

[github-actions]

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