Created OpenXR Skybox (markdown)

OpenXR-Skybox.md

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+WlxOverlay-S comes with a skybox for OpenXR. 
+
+# Changing the Skybox
+
+A custom texture may be set via `~/.config/wlxoverlay/conf.d`
+```yaml
+skybox_texture = starry-sky.dds
+```
+
+Criteria:
+- Path can be absolute, or relative to `~/.config/wlxoverlay/`.
+- File must be DDS (See: [Custom Textures](https://github.com/galister/wlx-overlay-s/wiki/Custom-Textures))
+- Image must be an equirectangular (aka "HDRI" or "Spherical 360") image.
+
+# Disable the skybox
+
+```bash
+use_skybox = false
+```