chore(gatsby-transformer-xml): Update README

[violetamenendez]

Re-sent this PR as it wasn't passing the linting tests (I edited it in github directly, my bad)

Description

Updating the documentation with complete examples of the queries and the results. As well as updating some comments for better understanding of how the plugin works.

Related Issues

Fixes #13773

[LekoArts]

Thank you! 👍

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