docs(bundler): add short usage in readme

Author: posva

packages/@posva/vuefire-bundler/README.md

@@ -1,11 +1,23 @@
 # `@posva/vuefire-bundler`
 
-> TODO: description
+> common config used to bundle vuefire and vuexfire
 
 ## Usage
 
+```sh
+node --eval 'require("@posva/vuefire-bundler")("MyLibrary")'
 ```
-const vuefireBundler = require('@posva/vuefire-bundler');
 
-// TODO: DEMONSTRATE API
+It can be added as a script in `package.json` file:
+
+```json
+{
+  "scripts": {
+    "build": "node --eval 'require(\"@posva/vuefire-bundler\")(\"Vuexfire\")'"
+  }
+}
 ```
+
+## License
+
+[MIT](http://opensource.org/licenses/MIT)