Use enhanced barriers on n Body Gravity sample (#937)

* Converted nBodyGravity to use enhanced barriers

* Update readme

Author: alecardoso-tech

Samples/Desktop/D3D12nBodyGravity/readme.md

@@ -18,4 +18,6 @@ extendedZipContent:
 This sample demonstrates the use of asynchronous compute shaders (multi-engine) to simulate an n-body gravity system. Graphics commands and compute commands can be recorded simultaneously, and submitted to their respective command queues when the work is ready to begin execution on the GPU. This sample also demonstrates advanced usage of fences to synchronize tasks across command queues.
 
 ### Optional features
-This sample has been updated to build against the Windows 10 Anniversary Update SDK. In this SDK a new revision of Root Signatures is available for Direct3D 12 apps to use. Root Signature 1.1 allows for apps to declare when descriptors in a descriptor heap won't change or the data descriptors point to won't change.  This allows the option for drivers to make optimizations that might be possible knowing that something (like a descriptor or the memory it points to) is static for some period of time.
+This sample has been updated to build against the Windows 10 Anniversary Update SDK. In this SDK a new revision of Root Signatures is available for Direct3D 12 apps to use. Root Signature 1.1 allows for apps to declare when descriptors in a descriptor heap won't change or the data descriptors point to won't change.  This allows the option for drivers to make optimizations that might be possible knowing that something (like a descriptor or the memory it points to) is static for some period of time.
+
+This sample uses Enhanced Barriers if the hardware supports it. Otherwise, Legacy Barriers will be used instead.