Add note about multiple files

ci/environment-mindeps.yaml

@@ -1,3 +1,7 @@
+# NOTE: Minimum dependencies are defined in `pyproject.toml`.
+# This file is only used for testing purposes.
+# When making changes to minimum dependencies, make sure both this file
+# and `pyproject.toml` are consistent with each other.
 name: test-environment
 channels:
   - conda-forge

pyproject.toml

@@ -29,6 +29,9 @@ classifiers = [
     "License :: OSI Approved :: MIT License",
 ]
 
+# NOTE: Minimum dependencies are tested with the packages defined in the
+# `ci/environment-mindeps.yaml` conda environment. When updating minimum dependencies
+# here, make sure to also update `ci/environment-mindeps.yaml`.
 [tool.poetry.dependencies]
 python = ">=3.8,<4.0"
 python-cmr = ">=0.9.0"