Update transform-object-assign's README from babel.github.io [skip ci] (#4929)

packages/babel-plugin-transform-object-assign/README.md

@@ -1,6 +1,6 @@
 # babel-plugin-transform-object-assign
 
-Replace `Object.assign` with an inline helper.
+> Replace `Object.assign` with an inline helper. If you are authoring an application, rather than a library, it is recommended that you use the `Object.assign` polyfill instead.
 
 ## Example
 
@@ -18,6 +18,15 @@ var _extends = ...;
 _extends(a, b);
 ```
 
+## Caveats
+
+- Will only work with code of the form `Object.assign` or `Object['assign']`. The following patterns are not supported:
+
+  ```javascript
+  var { assign } = Object;
+  var assign = Object.assign;
+  ```
+
 ## Installation
 
 ```sh