docs: add readme for each source dir

data/README.md

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+# Nextclade datasets - sources root
+
+This is the root of Nextclade dataset sources.
+
+You can add, remove and modify datasets here, then submit them in the form of a pull request for potential inclusion to the list of Nextclade datasets. Please read [Dataset curation guide](../docs/dataset-curation-guide.md) for instructions.
+
+The relative paths of each dataset in leaf subdirectories is used as a unique identifier of the dataset.
+
+For all questions and bug reports, please reach out to Nextstrain team using [GitHub issues](https://github.com/nextstrain/nextclade_data/issues).

data/community/README.md

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+# Nextclade datasets - community collection
+
+This directory contains source files of Nextclade datasets comprising Nextclade community dataset collection.
+
+These datasets are created and maintained by Nextclade community members. Nextclade team cannot provide support for these datasets or to guarantee their correctness. Please reach out to dataset authors for all details.
+
+You are free to contribute to the existing datasets (by arrangement with the dataset authors) or to submit new datasets. Please read [Dataset curation guide](../../docs/dataset-curation-guide.md) for instructions.
+
+Please report broken datasets to Nextclade team by submitting a [GitHub issue](https://github.com/nextstrain/nextclade_data/issues).

data/nextstrain/README.md

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+# Nextclade datasets - Nextstrain collection
+
+This directory contains source files of Nextclade datasets comprising "official" collection of Nextclade datasets.
+
+These datasets are created and maintained by Nextstrain team. For all questions and bug reports, please reach out to Nextstrain team using [GitHub issues](https://github.com/nextstrain/nextclade_data/issues).