Update README

Author: ericf

README.md

@@ -1,6 +1,8 @@
 # babel-plugin-react-intl
 
-Extracts string messages from modules that use react-intl.
+**THIS IS A PREVIEW RELEASE THAT WORKS WITH A FUTURE, UNRELEASED VERSION OF REACT INTL.**
+
+Extracts string messages from React components that use [React Intl][].
 
 ## Installation
 
@@ -16,10 +18,17 @@ $ npm install babel-plugin-react-intl
 
 ```json
 {
-  "plugins": ["react-intl"]
+  "plugins": ["react-intl"],
+  "extra": {
+    "react-intl": {
+        "messagesDir": "./build/messages/"
+    }
+  }
 }
 ```
 
+The `messagesDir` option is the target location where the plugin will output a `.json` file corresponding to each component from which React Intl messages were extracted.
+
 ### Via CLI
 
 ```sh
@@ -31,5 +40,8 @@ $ babel --plugins react-intl script.js
 ```javascript
 require("babel-core").transform("code", {
   plugins: ["react-intl"]
-});
+}) // => { code, map, ast, metadata['react-intl'].messages };
 ```
+
+
+[React Intl]: http://formatjs.io/react/