[eslint config] [react] Add guide section on react/jsx-curly-spacing

Per #693.

react/README.md

@@ -184,6 +184,16 @@
     <Foo />
     ```
 
+  - Do not pad JSX curly braces with spaces. eslint: [`react/jsx-curly-spacing`](https://github.com/yannickcr/eslint-plugin-react/blob/master/docs/rules/jsx-curly-spacing.md)
+
+    ```jsx
+    // bad
+    <Foo bar={ baz } />
+
+    // good
+    <Foo bar={baz} />
+    ```
+
 ## Props
 
   - Always use camelCase for prop names.