Update React transform READMEs from babel.github.io [skip ci] (#4903)

* Update React transform READMEs from babel.github.io [skip ci]

* Drop inline-elements polyfill message (move to website)

packages/babel-plugin-transform-react-inline-elements/README.md

@@ -1,11 +1,46 @@
 # babel-plugin-transform-react-inline-elements
 
-Turn JSX elements into exploded React objects
+Replaces the `React.createElement` function with one that is more optimized for production: `babelHelpers.jsx`.
+
+This transform **should be enabled only in production** (e.g., just before minifying your code) because although it improves runtime performance, it makes warning messages more cryptic and skips important checks that happen in development mode, including propTypes.
+
+## Example
+
+**In**
+
+```javascript
+<Baz foo="bar" key="1"></Baz>;
+```
+
+**Out**
+
+```javascript
+babelHelpers.jsx(Baz, {
+  foo: "bar"
+}, "1");
+
+/**
+ * Instead of
+ *
+ * React.createElement(Baz, {
+ *   foo: "bar",
+ *   key: "1",
+ * });
+ */
+```
+
+**Deopt**
+
+```js
+// The plugin will still use React.createElement when `ref` or `object rest spread` is used
+<Foo ref="bar" />
+<Foo {...bar} />
+```
 
 ## Installation
 
 ```sh
-$ npm install babel-plugin-transform-react-inline-elements
+npm install babel-plugin-transform-react-inline-elements
 ```
 
 ## Usage
@@ -23,7 +58,7 @@ $ npm install babel-plugin-transform-react-inline-elements
 ### Via CLI
 
 ```sh
-$ babel --plugins transform-react-inline-elements script.js
+babel --plugins transform-react-inline-elements script.js
 ```
 
 ### Via Node API
@@ -33,3 +68,7 @@ require("babel-core").transform("code", {
   plugins: ["transform-react-inline-elements"]
 });
 ```
+
+## References
+
+* [[facebook/react#3228] Optimizing Compiler: Inline React Elements](https://github.com/facebook/react/issues/3228)