docs: add section to readme

Author: G-Rath

README.md

@@ -1614,6 +1614,29 @@ This generates:
 </form>
 ```
 
+Additional options will be passed on as attributes to the root `div`:
+
+```erb
+<%= bootstrap_form_for @user_with_error do |f| %>
+  <%= f.alert_message "Please fix the errors below.", tabindex: -1, role: "note" %>
+<% end %>
+```
+
+This generates:
+
+```html
+<form accept-charset="UTF-8" action="/users" class="new_user" id="new_user" method="post">
+  <div class="alert alert-danger" tabindex="-1" role="note">
+    <p>Please fix the errors below.</p>
+    <ul class="rails-bootstrap-forms-error-summary">
+      <li>Email is invalid</li>
+      <li>Misc is invalid</li>
+      <li>Preferences is invalid</li>
+    </ul>
+  </div>
+</form>
+```
+
 To output a simple unordered list of errors, use the `error_summary` helper.
 
 ![Example 51](demo/doc/screenshots/bootstrap/readme/51_example.png "Example 51")