Update docs.html

Added heads up regarding lone icons providing accessibility issues for screen readers.

docs/docs.html

@@ -2561,6 +2561,11 @@ For example, <code>&lt;section&gt;</code> should be wrapped as inline.
 </div>
 {% endhighlight %}
 
+    <p class="alert alert-info">
+      <strong>Heads up!</strong>
+      Using icons without any additional content means the icons are not represented to screen reader users.
+    </p>
+
     <h5>Dropdown in a button group</h5>
     <div class="bs-docs-example">
       <div class="btn-group">