add xarray-schema

[jhamman]

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[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipes/xarray-schema) and found it was in an excellent condition.

[jhamman]

@conda-forge/staged-recipes - this is ready to merge

[jhamman]

Thanks @synapticarbors!