Add information about environment variables in matplotlib.__doc__

Author: oscargus

lib/matplotlib/__init__.py

@@ -77,6 +77,24 @@
         figure is created, because it is not possible to switch between
         different GUI backends after that.
 
+The following environment variables can be used to customize the behavior::
+
+    .. envvar:: MPLBACKEND
+
+      This optional variable can be set to choose the Matplotlib backend. See
+      :ref:`what-is-a-backend`.
+
+    .. envvar:: MPLCONFIGDIR
+
+      This is the directory used to store user customizations to
+      Matplotlib, as well as some caches to improve performance. If
+      :envvar:`MPLCONFIGDIR` is not defined, :file:`{HOME}/.config/matplotlib`
+      and :file:`{HOME}/.cache/matplotlib` are used on Linux, and
+      :file:`{HOME}/.matplotlib` on other platforms, if they are
+      writable. Otherwise, the Python standard library's `tempfile.gettempdir`
+      is used to find a base directory in which the :file:`matplotlib`
+      subdirectory is created.
+
 Matplotlib was initially written by John D. Hunter (1968-2012) and is now
 developed and maintained by a host of others.