readme: add simple description and screenshot

Author: willnorris

README.md

@@ -1,5 +1,10 @@
-<!-- README.md auto-generated by misc/genreadme; DO NOT EDIT. (or remove this line) -->
-
 # golink
 
-The golink server runs http://go/, a private shortlink service for tailnets.
+[![status: experimental](https://img.shields.io/badge/status-experimental-blue)](https://tailscale.com/kb/1167/release-stages/#experimental)
+
+golink is a private shortlink service for your [tailnet].  It lets you create
+short, memorable links for the websites you and your team use most.
+
+[tailnet]: https://tailscale.com/kb/1136/tailnet/
+
+![Screenshot of golink home screen](screenshot.png)