docs: add documentation

docs/cli.md

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+# CLI
+
+```sh title="nix run .#nixtests:run -- --help"
+Usage of nixtest:
+      --junit string          Path to generate JUNIT report to, leave empty to disable
+      --pure                  Unset all env vars before running script tests
+      --skip string           Regular expression to skip (e.g., 'test-.*|.*-b')
+      --snapshot-dir string   Directory where snapshots are stored (default "./snapshots")
+      --tests string          Path to JSON file containing tests
+      --update-snapshots      Update all snapshots
+      --workers int           Amount of tests to run in parallel (default 4)
+```