Added configuration section.

README.md

@@ -23,3 +23,26 @@ How to run
 % bundle install
 % bundle exec trema run lib/patch_panel.rb -c trema.conf
 ```
+
+
+Configuration
+-------------
+
+When `patch_panel.rb` starts up, it will search for its configuration
+file named `patch_panel.conf` in the current working directory. If you
+want to specify a path to a configuration file, pass it to
+`patch_panel.rb` as its argument:
+
+```
+% bundle exec trema run "lib/patch_panel.rb foobar.conf" -c trema.conf
+```
+
+A configuration file contains patching information. Each line has a
+pair of port numbers like so:
+
+```
+1 2
+3 4
+```
+
+The first line means that switch port #1 is connected to port #2.