Fix #2633 - Document user level config (#2736)

Author: ziima

docs/changelog/2633.doc.rst

@@ -0,0 +1 @@
+Document user level config.

docs/config.rst

@@ -750,3 +750,16 @@ Pip installer
    If ``true``, adds ``--pre`` to the ``opts`` passed to :ref:`install_command`. This will cause it to install the
    latest available pre-release of any dependencies without a specified version. If ``false``, pip will only install
    final releases of unpinned dependencies.
+
+User configuration
+------------------
+
+tox allows creation of user level config-file to modify default values of the CLI commands.
+It is localted in the OS-specific user config directory under ``tox/config.ini`` path, see ``tox --help`` output for exact location.
+It can be changed via ``TOX_CONFIG_FILE`` environment variable.
+Example configuration:
+
+.. code-block:: ini
+
+    [tox]
+    skip_missing_interpreters = true