Add image format footer explanation

Fixes: https://github.com/resin-io/resin-etcher/issues/86

build/css/main.css

@@ -5993,3 +5993,6 @@ body {
   margin-bottom: -40px;
   color: #85898c;
   font-size: 12px; }
+
+.tiny {
+  font-size: 10px; }

lib/index.html

@@ -32,6 +32,8 @@
                 <div ng-show="app.selection.hasImage()">
                   <span ng-bind="app.selection.getImage() | basename"></span>
                 </div>
+
+                <p class="step-footer tiny">*supported files: .img, .iso</p>
               </div>
             </div>
           </div>

lib/scss/main.scss

@@ -151,3 +151,7 @@ body {
   color: lighten($color-disabled, 5%);
   font-size: 12px;
 }
+
+.tiny {
+  font-size: 10px;
+}