[XR] Update README for 2018.3

Also clean up some of the language, which either did not make sense or was awkwardly worded.

README.md

 # AR Foundation Samples
-Example projects that utilize *AR Foundation* and demonstrates its functionality with sample assets and components.
+Example projects that use *AR Foundation* and demonstrate its functionality with sample assets and components.
-Requires Unity version 2018.1+
-
-Requires the AR Foundation package
+Compatible with Unity 2018.3.
-1. Download the latest version of Unity 2018.1 or 2018.2
+1. Download the latest version of Unity 2018.3
-3. Open your choice of sample scenes.
+3. Open your choice of sample scene.
-4. See the documentation for the AR Foundation for usage instructions and more information
-See the [AR Foundation Documentation](https://docs.unity3d.com/Packages/com.unity.xr.arfoundation@1.0/manual/index.html) to learn how to get started with the core components.
+4. See the [AR Foundation Documentation](https://docs.unity3d.com/Packages/com.unity.xr.arfoundation@1.0) for usage instructions and more information.