Added note about install_requires

Author: astrofrog

setup.cfg

@@ -23,6 +23,10 @@ long_description = file: README.rst
 [options]
 zip_safe = True
 packages = find:
+# Note that we don't include Matplotlib as a required dependency since it is only
+# required if the plugin is actually used, in which case the package using this
+# will already need to have Matplotlib declared as a dependency. We want to avoid
+# installing Matplotlib unecessarily.
 install_requires =
     pytest