Add doc page for WARP configuration (#315)

This adds a simple doc page describing the two useful options for
configuring WARP.

Author: llvm-beanz

docs/WARP.md

@@ -0,0 +1,22 @@
+# Using WARP on Windows
+
+Windows has a software rasterizer implementation called the Windows Advanced
+Rasterization Platform (WARP), which is a conforming implementation of DirectX
+and Vulkan 1.2.
+
+The offload-test-suite supports running against WARP on Windows by default and
+will generate test targets for `check-hlsl-warp-d3d12` and
+`check-hlsl-clang-warp-d3d12`.
+
+There are some useful CMake options to tweak the configuration to better utilize
+WARP:
+
+* **OFFLOADTEST_WARP_ONLY** - Skips generating d3d test configurations for
+  non-WARP configurations. This is useful if you're running Windows in a VM and
+  do not have a physical GPU.
+
+* **WARP_VERSION** - Defaults to `System` which uses the system version of WARP.
+  This option may also be set to an explicit WARP version, and the configuration
+  step will pull WARP from NuGet. See the [NuGet package
+  listing](https://www.nuget.org/packages/Microsoft.Direct3D.WARP) to identify
+  valid versions.