[all] Add desktop availability to the README.

[cbenhagen]

Description

Add desktop availability column to the README.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• If the pull request affects only one plugin, the PR title starts with the name of the plugin in brackets (e.g. [cloud_firestore])
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• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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• I read and followed the Flutter Style Guide.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
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Breaking Change

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• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[ditman]

LGTM! Not sure how we're going to scale up that table as new platforms get added! :P

[cbenhagen]

@ditman I hope we will soon face this problem ;) I am looking forward to Linux and Windows versions of the plugins.