Docs: add link

css_flexbox/index.html

@@ -18,7 +18,9 @@ <h1> Flexbox.</h1>
       <p>CSS Flexible Box Layout is intended for layout of items in one dimension. In the flex layout model, the children of a flex container can be laid out in any direction, and can “flex” their sizes, either growing to fill unused space or shrinking to avoid overflowing the parent. Both horizontal and vertical alignment of the children can be easily manipulated.</p>
       <p>Though you <em>could</em> create entire page layouts with flexbox, it's most suitable for working with interface elements and smaller components.</p>
       <p><a href="https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout">Mozillas guide on Flexbox Layouts</a><br>
-        <a href="https://demos.scotch.io/visual-guide-to-css3-flexbox-flexbox-playground/demos/">A Flexbox playground.</a></p>
+        <a href="https://demos.scotch.io/visual-guide-to-css3-flexbox-flexbox-playground/demos/">A Flexbox playground.</a><br>
+      <a href="https://css-tricks.com/snippets/css/a-guide-to-flexbox/">CSS-Tricks Complete Guide to Flexbox</a>
+      </p>
     </div>
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       <p>Hi</p>