add a quickie plugins/ dir readme mentioning how to add plugins, plus ref to wiki pages

plugins/README-plugins

+Several example plugins are included in the plugins/ directory. You
+can enable a plugin with the following line in config.php:
+
+    addPlugin('Example', array('param1' => 'value1',
+                               'param2' => 'value2'));
+
+This will look for and load files named 'ExamplePlugin.php' or
+'Example/ExamplePlugin.php' either in the plugins/ directory (for
+plugins that ship with StatusNet) or in the local/ directory (for
+plugins you write yourself or that you get from somewhere else) or
+local/plugins/.
+
+Plugins are documented in their own directories.
+
+
+Additional information on using and developing plugins can be found
+on the StatusNet wiki:
+
+http://status.net/wiki/Plugins
+http://status.net/wiki/Plugin_development
+