Add instructions on how to access the releases gallery feed inside VS

Author: shana

README.md

@@ -5,6 +5,10 @@ Most of the extension UI lives in the Team Explorer pane, which is available fro
 
 Official builds of this extension are available at [the official website](https://visualstudio.github.com).
 
+Older and pre-release/beta/untested versions are available at [the releases page](https://github.com/github/VisualStudio/releases), and also via a custom gallery feed for Visual Studio.
+
+You can configure the gallery by going to `Tools / Options / Extensions and Updates` and adding a new gallery with the url https://visualstudio.github.com/releases/feed.rss. The gallery will now be available from `Tools / Extensions and Updates`.
+
 [![Join the chat at https://gitter.im/github/VisualStudio](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/github/VisualStudio?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 ## Build requirements