Update event-receiver-and-list-event-receiver-sharepoint-add-in.md

Creating a new branch to match the naming standards of other branches.  Also added the description field for the doc with a summary of the page contents.  
The previous patch-1 branch including the updated link for [Overview of SharePoint webhooks]

Author: pstakuu

docs/solution-guidance/event-receiver-and-list-event-receiver-sharepoint-add-in.md

@@ -2,6 +2,7 @@
 title: Event receivers and list event receivers in the SharePoint Add-in model
 ms.date: 04/22/2020
 localization_priority: Priority
+description: High level overview and examples of how event receivers work in the new SharePoint Add-in model
 ---
 Event receivers and list event receivers in the SharePoint Add-in model
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