Update README-template.md

Author: hugoabernier

samples/README-template.md

@@ -3,7 +3,13 @@
 ## Summary
 Short summary on functionality and used technologies.
 
-[picture of the extension in action, if possible]
+> Please provide a high-quality screenshot of your extension below. It should be stored in a folder called `assets`. 
+> If possible, use a resolution of 1920x1080. 
+> If your extension requires the user to configure it, please use a screenshot of the extension as it appears after it has been configured.
+> You can add as many screen shots as you'd like to help users understand your extension without having to download it and install it.
+> DELETE THIS PARAGRAPH BEFORE SUBMITTING
+
+![picture of the extension in action](assets/preview.png)
 
 ## Used SharePoint Framework Version 
 ![drop](https://img.shields.io/badge/version-GA-green.svg)
@@ -63,4 +69,4 @@ Here's a debug URL for testing around this sample. **Updated based on your manif
 
 > Notice that better pictures and documentation will increase the sample usage and the value you are providing for others. Thanks for your submissions advance.
 
-<img src="https://telemetry.sharepointpnp.com/sp-dev-fx-extensions/samples/readme-template" />
+<img src="https://telemetry.sharepointpnp.com/sp-dev-fx-extensions/samples/readme-template" />