Add CLAUDE.md with development setup instructions (#10692)

Simple development instructions for using uv with xarray, including
commands for running tests and linting.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <[email protected]>

Author: max-sixty

CLAUDE.md

@@ -0,0 +1,21 @@
+# xarray development setup
+
+## Setup
+
+```bash
+uv sync
+```
+
+## Run tests
+
+```bash
+uv run pytest xarray -n auto  # All tests in parallel
+uv run pytest xarray/tests/test_dataarray.py  # Specific file
+```
+
+## Linting & type checking
+
+```bash
+pre-commit run --all-files  # Includes ruff and other checks
+uv run dmypy run  # Type checking with mypy
+```