added bootstrap

public/bootstrap-4.0.0/_includes/callout-danger-async-methods.md

@@ -0,0 +1,7 @@
+{% callout danger %}
+#### Asynchronous methods and transitions
+
+All API methods are **asynchronous** and start a **transition**. They return to the caller as soon as the transition is started but **before it ends**. In addition, a method call on a **transitioning component will be ignored**.
+
+[See our JavaScript documentation for more information.]({{ site.baseurl }}/docs/{{ site.docs_version }}/getting-started/javascript/)
+{% endcallout %}