docs: add README

README.md

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+# TCGA Downloader
+
+Python package + CLI to query public TCGA files and download via gdc-client.
+
+## Install
+
+```bash
+pip install -e .
+```
+
+## Example
+
+```bash
+tcga-downloader query --project TCGA-BRCA --data-type "Gene Expression" --out manifest.tsv
+
+tcga-downloader download --manifest manifest.tsv --out-dir ./data
+```

tests/test_readme.py

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+from pathlib import Path
+
+
+def test_readme_present():
+    assert Path("README.md").exists()