Some documentation improvements (#742)

* Link configuration examples in the intro, see #712

* Clarify overriding conf files by options, see #678

README.rst

@@ -172,6 +172,11 @@ i.e. ``C:\Users\<username>\``)
 
 Values in later configuration files will override previous ones.
 
+Command line options will override all related settings in the configuration file(s),
+e.g. using ``--write-metadata`` will enable writing metadata using the default values
+for all ``postprocessors.metadata.*`` settings, overriding any specific settings in
+configuration files.
+
 
 Authentication
 ==============

docs/configuration.rst

@@ -1,6 +1,15 @@
 Configuration
 #############
 
+Configuration files for *gallery-dl* use a JSON-based file format.
+
+For a (more or less) complete example with options set to their default values, see gallery-dl.conf_.
+
+For a configuration file example with more involved settings and options, see gallery-dl-example.conf_.
+
+This file lists all available configuration options and their descriptions.
+
+
 Contents
 ========
 
@@ -2151,6 +2160,8 @@ Description An object with the ``name`` of a post-processor and its options.
 .. _date-format: `extractor.*.date-format`_
 .. _deviantart.metadata: extractor.deviantart.metadata_
 
+.. _gallery-dl.conf:         https://github.com/mikf/gallery-dl/blob/master/docs/gallery-dl.conf
+.. _gallery-dl-example.conf: https://github.com/mikf/gallery-dl/blob/master/docs/gallery-dl-example.conf
 .. _.netrc:             https://stackoverflow.com/tags/.netrc/info
 .. _Last-Modified:      https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.29
 .. _datetime:           https://docs.python.org/3/library/datetime.html#datetime-objects