add notices and warnings about the testdata env vars

CONTRIBUTING.rst

@@ -106,6 +106,14 @@ Ready to contribute? Here's how to set up ``xhydro`` for local development.
     # Or, to run multiple build tests
     $ tox
 
+    .. note::
+
+        Running `pytest` or `tox` will automatically fetch and cache the testing data for the package to your local cache (using the `platformdirs` library). On Linux, this is located at ``XDG_CACHE_HOME`` (usually ``~/.cache``). On Windows, this is located at ``%LOCALAPPDATA%`` (usually ``C:\Users\username\AppData\Local``). On MacOS, this is located at ``~/Library/Caches``.
+
+        If for some reason you wish to cache this data elsewhere, you can set the ``XHYDRO_DATA_DIR`` environment variable to a different location before running the tests. For example, to cache the data in the current working directory, run::
+
+            $ export XHYDRO_DATA_DIR=$(pwd)/.cache
+
 #. Commit your changes and push your branch to GitHub::
 
     $ git add .
@@ -134,6 +142,12 @@ Ready to contribute? Here's how to set up ``xhydro`` for local development.
 
 You will have contributed your first changes to ``xhydro``!
 
+.. warning::
+
+    If your Pull Request relies on modifications to the testing data of `xhydro`, you will need to update the testing data repository as well. As a preliminary testing measure, the branch of the testing data can be modified at testing time (from `main`) by setting the ``XHYDRO_TESTDATA_BRANCH`` environment variable to the branch name of the ``xhydro-testdata`` repository.
+
+    Be sure to consult the ReadMe found at https://github.com/hydrologie/xhydro-testdata as well.
+
 Pull Request Guidelines
 -----------------------