guide: import/extensions rule documentation

README.md

@@ -1441,6 +1441,23 @@ Other Style Guides
     import barCss from 'bar.css';
     ```
 
+  <a name="modules--import-extensions"></a>
+  - [10.10](#modules--import-extensions) Do not include JavaScript filename extensions
+ eslint: [`import/extensions`](https://github.com/benmosher/eslint-plugin-import/blob/master/docs/rules/extensions.md)
+    > Why? Including extensions inhibits refactoring, and inappropriately hardcodes implementation details of the module you're importing in every consumer.
+
+    ```javascript
+    // bad
+    import foo from './foo.js';
+    import bar from './bar.jsx';
+    import baz from './baz/index.jsx';
+
+    // good
+    import foo from './foo';
+    import bar from './bar';
+    import baz from './baz';
+    ```
+
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 ## Iterators and Generators