Update transform-es2015 READMEs from babel.github.io [skip ci] (#4926)

Signed-off-by: Brian Ng <bng412@gmail.com>

packages/babel-plugin-transform-es2015-modules-commonjs/README.md

@@ -1,5 +1,25 @@
 # babel-plugin-transform-es2015-modules-commonjs
 
+> This plugin transforms ES2015 modules to [CommonJS](http://wiki.commonjs.org/wiki/Modules/1.1).
+
+## Example
+
+**In**
+
+```javascript
+export default 42;
+```
+
+**Out**
+
+```javascript
+Object.defineProperty(exports, "__esModule", {
+  value: true
+});
+
+exports.default = 42;
+```
+
 ## Installation
 
 ```sh
@@ -41,3 +61,27 @@ require("babel-core").transform("code", {
   plugins: ["transform-es2015-modules-commonjs"]
 });
 ```
+
+## Options `loose`
+
+As per the spec, `import` and `export` are only allowed to be used at the top
+level. When in loose mode these are allowed to be used anywhere.
+
+And by default, when using exports with babel a non-enumerable `__esModule` property
+is exported.
+
+```javascript
+var foo = exports.foo = 5;
+
+Object.defineProperty(exports, "__esModule", {
+  value: true
+});
+```
+
+In environments that don't support this you can enable loose mode on `es6.modules`
+and instead of using `Object.defineProperty` an assignment will be used instead.
+
+```javascript
+var foo = exports.foo = 5;
+exports.__esModule = true;
+```