docs: Make it explicit that people shouldn't use @latest with npmcdn

Author: trodrigues

README.md

@@ -50,13 +50,16 @@ Or, if you'd like to use a standalone built file you can use the following scrip
 ``` html
 <script src="https://npmcdn.com/contentful@latest/browser-dist/contentful.min.js"></script>
 ```
+**It is not recommended to use the above URL for production.**
 
 Using `contentful@latest` will always get you the latest version, but you can also specify a specific version number:
 
 ``` html
 <script src="https://npmcdn.com/contentful@3.0.0/browser-dist/contentful.min.js"></script>
 ```
 
+Check the [releases](https://github.com/contentful/contentful.js/releases) page to know which versions are available.
+
 ## Authentication
 
 To get content from Contentful, an app should authenticate with an with an OAuth bearer token.