Add docs for config file [skip ci]

Author: ulope

docs/config_file.rst

@@ -0,0 +1,66 @@
+Raiden Config File
+##################
+
+Raiden supports reading configuration parameters from a configuration file.
+
+Location
+--------
+
+The default location is ``<datadir>/config.toml`` (where datadir defaults to ``~/.raiden``).
+
+It is possible to override the ``datadir`` in the config file but please be aware that at that point
+the config file will already have been loaded from the default location and therefore won't be
+re-read from the 'new' datadir.
+
+Precedence
+----------
+
+The precedence order in which configuration option values are applied is as follows (high to low):
+
+#. Option given on the command line
+#. Option read from the config file
+#. Option default value (as seen in the output of ``raiden --help``)
+
+
+Format
+------
+
+The config file uses the `TOML`_ format.
+
+Option names may be quoted unless they contain punctuation in which case they must be.
+Values except numbers must always be quoted. Both single or double quotes are acceptable.
+
+Lines *starting* with a ``#`` are comments.
+
+.. _TOML: https://github.com/toml-lang/toml
+
+
+Parameter Naming
+----------------
+
+All parameters that can be given as command line options are also settable in the config file.
+The name corresponds to the long option name without the leading double dash (``--``). For example
+the CLI option ``--password-file`` would be called ``password-file`` inside the config file.
+
+The only option deviating from this scheme is the `logging configuration`_ which is explained
+below.
+
+
+Logging Configuration
+---------------------
+
+Raiden allows configuration of the logging system using a concise syntax on the command line.
+Inside the configuration file this is split out into a somewhat more expanded syntax.
+
+The logging configuration is placed inside a section called ``[log-config]`` which each following
+lines key representing the logger name and the value the log level.
+
+Example::
+
+    # CLI:
+    --log-config ':debug,raiden.network:info'
+
+    # Config file equivalent:
+    [log-config]
+    "" = "debug"
+    "raiden.network" = "info"

docs/index.rst

@@ -22,6 +22,7 @@ Contents
   webui_tutorial
   spec
   smart_contracts
+  config_file
   changelog
   glossary
   Raiden Codebase Documentation <./_build/generated/modules>