docs(input): add icon prefix/suffix example

Author: marshall007

src/demo-app/input/input-demo.html

@@ -54,11 +54,19 @@
 <md-card class="demo-card demo-basic">
   <md-toolbar color="primary">Prefix + Suffix</md-toolbar>
   <md-card-content>
+    <h4>Text</h4>
     <md-input-container align="end">
       <input mdInput placeholder="amount">
       <span mdPrefix>$&nbsp;</span>
       <span mdSuffix>.00</span>
     </md-input-container>
+
+    <h4>Icons</h4>
+    <md-input-container>
+      <input mdInput placeholder="amount">
+      <md-icon mdPrefix>attach_money</md-icon>
+      <md-icon mdSuffix>mode_edit</md-icon>
+    </md-input-container>
   </md-card-content>
 </md-card>