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-constant-elements/README.md

@@ -1,11 +1,54 @@
 # babel-plugin-transform-react-constant-elements
 
-Treat React JSX elements as value types and hoist them to the highest scope
+Hoists element creation to the top level for subtrees that are fully static, which reduces both allocations and calls to `React.createElement`. More importantly, it tells React that the subtree hasn't changed so React can completely skip it when reconciling.
+
+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.
+
+## Example
+
+**In**
+
+```js
+const Hr = () => {
+  return <hr className="hr" />;
+};
+```
+
+**Out**
+
+```js
+const _ref = <hr className="hr" />;
+
+const Hr = () => {
+  return _ref;
+};
+```
+
+**Deopts**
+
+- **Spread Operator**
+
+  ```js
+  <div {...foobar} />
+  ```
+
+- **Refs**
+
+  ```js
+  <div ref="foobar" />
+  <div ref={node => this.node = node} />
+  ```
+
+- **Composite Components**
+
+  ```js
+  const ComponentA = () => <div><MyCustomComponent /></div>;
+  ```
 
 ## Installation
 
 ```sh
-$ npm install babel-plugin-transform-react-constant-elements
+npm install babel-plugin-transform-react-constant-elements
 ```
 
 ## Usage
@@ -23,7 +66,7 @@ $ npm install babel-plugin-transform-react-constant-elements
 ### Via CLI
 
 ```sh
-$ babel --plugins transform-react-constant-elements script.js
+babel --plugins transform-react-constant-elements script.js
 ```
 
 ### Via Node API
@@ -33,3 +76,7 @@ require("babel-core").transform("code", {
   plugins: ["transform-react-constant-elements"]
 });
 ```
+
+## References
+
+* [[facebook/react#3226] Optimizing Compiler: Reuse Constant Value Types like ReactElement](https://github.com/facebook/react/issues/3226)