Add link to marketplace

Author: jjonescz

README.md

@@ -8,7 +8,9 @@ Studio](https://visualstudio.com/).
 
 ## Installation
 
-Download a `.vsix` from
+Install "Git Time Metric" from [Visual Studio
+Marketplace](https://marketplace.visualstudio.com/items?itemName=jjones.gtm.visualstudio.plugin).
+Alternatively, download a `.vsix` from
 [releases](https://github.com/jjonescz/gtm-visualstudio-plugin/releases) and run
 it.