Add example with visible <label> and input group addon

Also, include visible <label> as a suggestion in the accessibility callout. Addresses https://github.com/twbs/bootstrap/issues/15610#issuecomment-70578455

docs/_includes/components/input-groups.html

@@ -19,7 +19,7 @@
   <div class="bs-callout bs-callout-warning" id="callout-inputgroup-form-labels">
     <h4>Always add labels</h4>
     <p>Screen readers will have trouble with your forms if you don't include a label for every input. For these input groups, ensure that any additional label or functionality is conveyed to assistive technologies.</p>
-    <p>The exact technique to be used (<code>&lt;label&gt;</code> elements hidden using the <code>.sr-only</code> class, or use of the <code>aria-label</code>, <code>aria-labelledby</code>, <code>aria-describedby</code>, <code>title</code> or <code>placeholder</code> attribute) and what additional information will need to be conveyed will vary depending on the exact type of interface widget you're implementing. The examples in this section provide a few suggested, case-specific approaches.</p>
+    <p>The exact technique to be used (visible <code>&lt;label&gt;</code> elements, <code>&lt;label&gt;</code> elements hidden using the <code>.sr-only</code> class, or use of the <code>aria-label</code>, <code>aria-labelledby</code>, <code>aria-describedby</code>, <code>title</code> or <code>placeholder</code> attribute) and what additional information will need to be conveyed will vary depending on the exact type of interface widget you're implementing. The examples in this section provide a few suggested, case-specific approaches.</p>
   </div>
 
   <h2 id="input-groups-basic">Basic example</h2>
@@ -42,6 +42,12 @@
       <input type="text" class="form-control" aria-label="Amount (to the nearest dollar)">
       <span class="input-group-addon">.00</span>
     </div>
+    <br>
+    <label for="basic-url">Your vanity URL</label>
+    <div class="input-group">
+      <span class="input-group-addon" id="basic-addon3">https://example.com/users/</span>
+      <input type="text" class="form-control" id="basic-url" aria-describedby="basic-addon3">
+    </div>
   </form>
 {% highlight html %}
 <div class="input-group">
@@ -59,6 +65,12 @@
   <input type="text" class="form-control" aria-label="Amount (to the nearest dollar)">
   <span class="input-group-addon">.00</span>
 </div>
+
+<label for="basic-url">Your vanity URL</label>
+<div class="input-group">
+  <span class="input-group-addon" id="basic-addon3">https://example.com/users/</span>
+  <input type="text" class="form-control" id="basic-url" aria-describedby="basic-addon3">
+</div>
 {% endhighlight %}
 
   <h2 id="input-groups-sizing">Sizing</h2>