Adds explanatory comment

pyproject.toml

@@ -22,6 +22,8 @@ classifiers = [
 ]
 requires-python = ">=3.10"
 dependencies = [
+    # NOTE: Please also update dependency versions in pyaerocom_env.yml,
+    # and minimum versions in the tox config at the bottom of this file!
     "aerovaldb>=0.1.1",
     "scitools-iris>=3.11.0",
     "xarray>=2022.12.0",
@@ -216,6 +218,10 @@ extras =
 setenv =
     UDUNITS2_XML_PATH=/usr/share/xml/udunits/udunits2-common.xml
 deps =
+    # This pins key dependencies to the minimal supported version.
+    # Will only run on lowest supported Python version CI test 
+    # (currently 3.10).
+    # Should be incremented when changing dependencies above.  
     scitools-iris ==3.11.0; python_version < "3.11"
     cartopy ==0.21.1; python_version < "3.11"
     matplotlib ==3.7.1; python_version < "3.11"