docs(xo-server-test-plugin): explain configurationPresets

packages/xo-server-test-plugin/index.js

@@ -25,6 +25,15 @@ exports.configurationSchema = {
   required: ['foo'],
 }
 
+// This (optional) dictionary provides example configurations that can be used to help
+// configuring this plugin.
+//
+// The keys are the preset names, and the values are subset of the configuration.
+exports.configurationPresets = {
+  'preset 1': { foo: 'foo value 1' },
+  'preset 2': { foo: 'foo value 2' },
+}
+
 // This (optional) schema is used to test the configuration
 // of the plugin.
 exports.testSchema = {