Add a configuration file description in docs.

Give config file docs an own page.

docs/config.rst

+Configuration Files
+===================
+
+You can use configuration files to persistently override built-in
+defaults and make your custom settings the default.
+
+`diceware` configuration files follow simple ``.ini``-style and look
+like this::
+
+  [diceware]
+  num = 7
+  caps = off
+  specials = 2
+  delimiter = MYDELIMITER
+  randomsource = system
+  wordlist = en_8k
+
+The options names have to match long argument names, as output with
+``--help``. The values set must meet the requirements valid for
+commandline usage.
+
+You can use all or only some (or none) of the above options. Please
+note that other entries, providing unknown option names, are
+ignored. That means that also typos might lead to ignored entries.
+
+Please note, that all options must be set within a section
+``[diceware]``.
+
+
+Name and Path
+-------------
+
+Currently, we look for configuration files only in the calling users'
+home directory. The file must be called::
+
+ .diceware.ini
+
+(please note the leading dot). If such a file is missing, build-in
+defaults apply.
+
+
+Option Values
+-------------
+
+The option values set can be strings, integers, or boolean
+values.
+
+`diceware` accepts ``yes``, ``no``, ``1``, ``0``, ``true``, ``false``,
+``on``, and ``off`` as boolean values.
+
+Some options require their setting to be taken from a fixed set of
+names/values, for instance the ``randomsource`` option.
+
+If some value cannot be parsed, an exception is raised.

docs/index.rst

@@ -16,6 +16,7 @@ Version: |version|
 
    readme
    randomsources
+   config
    wordlists
    api
    changes