document Unity XR testing (fixes issue #155)

docs/xr-testing.md

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+# Enabling Virtual Reality player mode in Unity's XR Editor
+
+## XR Settings
+
+For rapid testing in your headset, you can enable Virtual Reality support in your [Unity project](./project-setup.md).
+
+![Animation of Unity's XR Settings](./images/unity-xr-settings.gif "Animation of Unity's XR Settings")
+
+1. Select the **`File > Build Settings…`** menu option from the top-level Unity application menu.
+2. When the pop-up window appears, select to highlight the **`PC, Mac & Linux Standalone`** option in the **`Platform`** list. (When you are ready to [build your application for WebVR](./build.md), ensure that **`WebGL`** is still the selected platform.)
+
+    ![Screenshot of "Platform" list in Unity's "Build Settings…" menu](./images/standalone-platform.png 'Screenshot of "Platform" list in Unity\'s "Build Settings…" menu')
+
+3. Click the **`Player Settings…`** button at the bottom of the pop-up window. Or you can access the **`Player Settings…`** by selecting the **`Edit > Project Settings > Player`** menu option from the top-level Unity application menu.
+
+    ![Screenshot of Unity's "Player Settings…" menu](./images/unity-xr-settings-5.png 'Screenshot of Unity\'s "Player Settings…" menu')
+
+4. In the **`Inspector`** pane, click the tab for **`Settings for PC, Mac & Linux Standalone`** pane (i.e., the icon with the underlined down arrow), if the tab is not already selected.
+
+    ![Screenshot of Unity's "Settings for PC, Mac & Linux Standalone" pane](./images/unity-xr-settings-6.png 'Screenshot of Unity\'s "Settings for PC, Mac & Linux Standalone" pane')
+
+5. Click on the **`XR Settings`** title to expose the section if it is hidden. Enable the **`Virtual Reality Supported`** checkbox field.
+
+    ![Screenshot of Unity's "XR Settings"](./images/unity-xr-settings-7.png 'Screenshot of Unity\'s "XR Settings"')
+
+6. Notice that are two **`Virtual Reality SDKs`** listed, sorted in the order in which Unity attempts to enable VR devices at runtime or during VR preview:
+    - **`OpenVR`** (i.e., for the HTC Vive and Windows Mixed Reality headsets)
+    - **`Oculus`** (i.e., for the Oculus Rift and, soon, Oculus Go headsets)
+
+    When building for [WebVR](./build.md), ensure that the **`WebGL`** option is still selected in the **`Platform`** list (**`File > Build Settings…`**).
+
+      ![Screenshot of "Platform" list in Unity's "Build Settings…" menu](./images/webgl-platform.png 'Screenshot of "Platform" list in Unity\'s "Build Settings…" menu')