Add Config docs section

Copying the Config section from Picos content-sample/index.md with some changes...

_docs/config.md

@@ -0,0 +1,36 @@
+---
+toc:
+    config:
+        _title: Config
+        url-rewriting: URL Rewriting
+---
+
+## Config
+
+You can override the default Pico settings (and add your own custom settings)
+by editing `config/config.php` in the Pico directory. For a brief overview of
+the available settings and their defaults see `config/config.php.template`. To
+override a setting, copy `config/config.php.template` to `config/config.php`,
+uncomment the setting and set your custom value.
+
+### URL Rewriting
+
+Picos default URLs (e.g. http://example.com/pico/?sub/page) already are very
+user friendly. Pico anyway offers you an URL rewrite feature to make URLs even
+more user friendly (e.g. http://example.com/pico/sub/page).
+
+If you're using the Apache web server, URL rewriting should be enabled
+automatically. If you get an error message from your web server, please make
+sure to enable the `mod_rewrite` module. Assumed rewritten URLs work, but Pico
+still shows no rewritten URLs, force URL rewriting by setting
+`$config['rewrite_url'] = true;` in your `config/config.php`.
+
+If you're using Nginx, you can use the following configuration to enable
+URL rewriting. Don't forget to adjust the path (`/pico/`; line `1` and `4`)
+to match your installation directory. You can then enable URL rewriting by
+setting `$config['rewrite_url'] = true;` in your `config/config.php`.
+
+    location /pico/ {
+        index index.php;
+        try_files $uri $uri/ /pico/?$uri&$args;
+    }