Added README.md to device/vr

Bug: 1099831
Change-Id: Ifd896b7caf8fedcf3083a8fc4ea6c8b1d85b94c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2290073
Commit-Queue: Brandon Jones <bajones@chromium.org>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786900}

device/vr/README.md

+# VR
+
+`device/vr` abstracts [WebXR](https://immersive-web.github.io/webxr/) features
+across multiple platforms.
+
+Although the directory is named "vr" both VR and AR features are implemented
+here. The name is an artifact from the time when it was associated with the now
+deprecated WebVR API.
+
+## Platform support
+
+Windows and Android are the primary supported platforms, largely because they're
+the only platforms with significant XR device traction at this point. Several
+native APIs are supported for various use cases, though things are trending
+towards OpenXR being the only API used on desktops.
+
+| API                   | OS       | Supports | Enabled by Default |
+|-----------------------|:--------:|:--------:|:------------------:|
+| OpenXR                | Windows* | VR*      | Yes**              |
+| Windows Mixed Reality | Windows  | VR       | Yes                |
+| AR Core               | Android  | AR       | Yes                |
+| Google VR             | Android  | VR       | Yes                |
+| Oculus                | Windows  | VR       | No                 |
+| OpenVR                | Windows  | VR       | No                 |
+
+ - * OpenXR may support multiple OSes and AR use cases as well. Currently we
+   only use it for VR on Windows since that's what the majority of existing
+   runtimes support.
+ - ** OpenXR runtimes are only enabled by default if they implement the
+   "[XR_EXT_win32_appcontainer_compatible](https://www.khronos.org/registry/OpenXR/specs/1.0/html/xrspec.html#XR_EXT_win32_appcontainer_compatible)"
+   extension.
+
+Integrations with some APIs (such as AR Core) are partially locaed in
+chrome/browser/vr due to architectural and historical limitations. In the future
+those will ideally be migrated to this directory as well.
+
+## Testing
+See [XR Browser Tests documentation](../../chrome/vr/test/xr_browser_tests.md).