Add documentation for the user config file

docs/terra/index.rst

@@ -11,3 +11,4 @@ Qiskit Terra Documentation
    Plotting Data in Qiskit <plotting_data_in_qiskit>
    Tools for Monitoring Backends <backend_monitoring_tools>
    Terra Parallel Tools <terra_parallel_tools>
+   Using User Config Files <user_config>

docs/terra/user_config.rst

@@ -0,0 +1,39 @@
+User Config Files
+=================
+
+When using Qiskit Terra there are several aspects of the library where Terra
+has opinionated defaults. These defaults are normally in the interest of
+supporting the least common demonitator between users. For example, the default
+backend for ``qiskit.visualization.circuit_drawer()``/``QuantumCircuit.draw()``
+is the text backend. However, depending on your local environment you may want
+to change these defaults to something better suited for your use case. This is
+what the user config file can be used for. It lets you optionally specify
+local defaults for these things
+
+
+Creating a User Config File
+---------------------------
+
+By default the user config file should be located in
+``~/.qiskit/settings.conf``. It is a ``.ini`` file that lists any options you'd
+like to change the defaults on. Right now there is only one supported option,
+``circuit_drawer`` which is used to set the default backend used when running
+the circuit drawer.
+
+For example, a ``settings.conf`` file with all the options set would look
+like::
+
+    [default]
+    circuit_drawer = latex
+
+In this file when you call the circuit drawer the default output will be
+``'latex'`` instead of ``'text'``.
+
+Alternative Locations for a User Config File
+--------------------------------------------
+
+The default location for the user config file is ``~/.qiskit/settings.conf``
+but if you want to store your user config file in a different location you
+can specify this with the ``QISKIT_SETTINGS`` environment variable. When
+``QISKIT_SETTINGS`` is set Qiskit Terra will try to use the value as the path
+to the user config file.