Add section 18.12 on line length

Author: banhathome

README.md

@@ -1692,6 +1692,31 @@ Other Style Guides
     // good
     const foo = { clark: 'kent' };
     ```
+  - [18.12](#18.12) <a name='18.12'></a> Avoid having lines of code that are longer than 100 characters (including whitespace).
+  > Why? This ensures readability and maintainability.
+
+  eslint rules: [`max-len`](http://eslint.org/docs/rules/max-len.html).
+
+    ```javascript
+  // bad
+  const foo = 'Whatever national crop flips the window. The cartoon reverts within the screw. Whatever wizard constrains a helpful ally. The counterpart ascends!';
+
+  // bad
+  $.ajax({ method: 'POST', url: 'https://airbnb.com/', data: { name: 'John' } }).done(() => console.log('Congratulations!')).fail(() => console.log('You have failed this city.'));
+
+  // good
+  const foo = 'Whatever national crop flips the window. The cartoon reverts within the screw. ' +
+    'Whatever wizard constrains a helpful ally. The counterpart ascends!';
+
+  // good
+  $.ajax({
+    method: 'POST',
+    url: 'https://airbnb.com/',
+    data: { name: 'John' },
+  })
+    .done(() => console.log('Congratulations!'))
+    .fail(() => console.log('You have failed this city.'));
+    ```
 
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