adding config file to user guide (#931)

* adding config file to user guide

* finished requested changes

Author: marcoslbueno

doc/usage.rst

@@ -29,6 +29,35 @@ the OpenML Python connector, followed up by a simple example.
 
 * `Introduction <examples/introduction_tutorial.html>`_
 
+~~~~~~~~~~~~~
+Configuration
+~~~~~~~~~~~~~
+
+The configuration file resides in a directory ``.openml`` in the home
+directory of the user and is called config. It consists of ``key = value`` pairs
+which are separated by newlines. The following keys are defined:
+
+* apikey:
+    * required to access the server. The `OpenML setup <https://openml.github.io/openml-python/master/examples/20_basic/introduction_tutorial.html#authentication>`_ describes how to obtain an API key.
+
+* server:
+    * default: ``http://www.openml.org``. Alternatively, use ``test.openml.org`` for the test server.
+
+* cachedir:
+    * if not given, will default to ``~/.openml/cache``
+
+* avoid_duplicate_runs:
+    * if set to ``True``, when ``run_flow_on_task`` or similar methods are called a lookup is performed to see if there already exists such a run on the server. If so, download those results instead.
+    * if not given, will default to ``True``.
+
+* connection_n_retries:
+    * number of connection retries.
+    * default: 2. Maximum number of retries: 20.
+
+* verbosity:
+    * 0: normal output
+    * 1: info output
+    * 2: debug output
 
 ~~~~~~~~~~~~
 Key concepts