chore: update doc

Author: jgtrescazes

website/docs/component-event.md

@@ -37,6 +37,7 @@ The following sections explain the properties of the domain object:
 -   [Touched at](#touched-at)
 -   [Summary](#summary)
 -   [Description](#description)
+-   [HTML Description](#html-description)
 -   [Occurrence](#occurrence)
 -   [Location](#location)
 -   [Organizer](#organizer)
@@ -113,6 +114,17 @@ $event = new Event();
 $event->setDescription('Lorem Ipsum Dolor...');
 ```
 
+### HTML Description
+
+In addition to the description, it is possible to add an alternative HTML description for clients that support it (ie. Outlook).
+
+```php
+use Eluceo\iCal\Domain\Entity\Event;
+
+$event = new Event();
+$event->setHtmlDescription('<p>Lorem Ipsum Dolor...</p>');
+```
+
 ### URL
 
 The URL can be used to link to an arbitrary resource.