README: Point to new xrgears.

README.md

+This is an old version of xrgears kept for archival purposes.
+The new OpenXR version can be found on the [Freedesktop Gitlab](https://gitlab.freedesktop.org/monado/demos/xrgears).
+
+
 ![Screenshot](https://gitlab.com/lubosz/xrgears/raw/master/doc/screenshot.jpg "Screenshot")
 
-# xrgears
+# xrgears-legacy
 
 xrgears is a stand alone VR demo using OpenHMD for tracking and Vulkan
 with several window system back ends for rendering.