add docs

README.md

@@ -322,7 +322,7 @@ ENV_NAME="cudf_test"
 rapids-dependency-file-generator \
   --file-key "test" \
   --output "conda" \
-  --matrix "cuda=11.5;arch=$(arch)" > env.yaml
+  --matrix "cuda=12.5;arch=$(arch)" > env.yaml
 mamba env create --file env.yaml
 mamba activate "$ENV_NAME"
 
@@ -335,6 +335,16 @@ The `--file-key`, `--output`, and `--matrix` flags must be used together. `--mat
 
 Where multiple values for the same key are passed to `--matrix`, e.g. `cuda_suffixed=true;cuda_suffixed=false`, only the last value will be used.
 
+Where `--file-key` is supplied multiple times in the same invocation, the output printed to `stdout` will contain a union (without duplicates) of all of the corresponding dependencies. For example:
+
+```shell
+rapids-dependency-file-generator \
+  --file-key "test" \
+  --file-key "test_notebooks" \
+  --output "conda" \
+  --matrix "cuda=12.5;arch=$(arch)" > env.yaml
+```
+
 The `--prepend-channel` argument accepts additional channels to use, like `rapids-dependency-file-generator --prepend-channel my_channel --prepend-channel my_other_channel`.
 If both `--output` and `--prepend-channel` are provided, the output format must be conda.
 Prepending channels can be useful for adding local channels with packages to be tested in CI workflows.

tests/test_rapids_dependency_file_generator.py

@@ -83,7 +83,7 @@ def test_make_dependency_file(mock_relpath):
     assert env == header + "dep1\ndep2\n"
 
 
-def test_make_dependency_files_should_raise_informative_error_when_extras_is_missing_for_pyproj():
+def test_make_dependency_file_should_raise_informative_error_when_extras_is_missing_for_pyproj():
 
     current_dir = pathlib.Path(__file__).parent
     with pytest.raises(ValueError, match=r"The 'extras' field must be provided for the 'pyproject' file type"):