Add section to readme explaining address plugin

ui-rsdir/README.md

@@ -3,3 +3,23 @@
 ## Introduction
 
 Front-end for the ReShare consortial directory
+
+## Address plugin configuration
+
+The address module uses `config.reshare.addressPlugin` from the Stripes
+configuration to select the plugin used to edit and display addresses. Supported
+values are `generic`, `north-america`, and `british-isles`. If the setting is
+omitted or has an unrecognized value, the module uses the `generic` plugin.
+
+For example, the relevant section of `stripes.config.js` for the North American
+address plugin is:
+
+```js
+module.exports = {
+  config: {
+    reshare: {
+      addressPlugin: 'north-america',
+    },
+  },
+};
+```