Add comment linking why we use appdirs in setup.cfg (#245)

Author: nathanjmcdougall

setup.cfg

@@ -32,6 +32,7 @@ install_requires =
     joblib>=0.12.0
     importlib-metadata>=4.4
     importlib-resources>=1.3
+    # Using appdirs rather than platformdirs is deliberate, see https://github.com/rstudio/pins-python/pull/239
     appdirs<2.0.0
     humanize>=1.0.0
     requests