Readme - register component when using CDN (#193)

* Readme - add a note about auto-registration in the browser

Author: LukaszWiktor

README.md

@@ -77,6 +77,13 @@ Download vue, luxon, weekstart and vue-datetime or use a CDN like unpkg.
 <script src="vue-datetime.js"></script>
 ```
 
+The component registers itself automatically as `<datetime>`. If you want to use a different name then register it explicitly:
+
+```js
+Vue.component('vue-datetime', window.VueDatetime.Datetime);
+```
+
+
 **weekstart** is optional, is used to get the first day of the week.
 
 ## Usage