Updated sample YAML description

Author: o365devx

README.md

@@ -1,3 +1,16 @@
+---
+topic: sample
+products:
+- office-excel
+- office-365
+languages:
+- javascript
+- html
+- typescript
+extensions:
+  contentType: samples
+  createdDate: 11/1/2017 4:03:37 PM
+---
 # Custom functions in Excel
 
 Custom functions enable you to add new functions to Excel by defining those functions in JavaScript as part of an add-in. Users within Excel can access custom functions just as they would any native function in Excel, such as `SUM()`.